Upgrade github-pages gem to fix deprecation warnings

Previously, when running bundle exec jekyll serve a false-positive
deprecation warning was reported:

Deprecation: Auto-regeneration can no longer be set from your
configuration file(s). Use the --[no-]watch/-w command-line option
instead.

See https://github.com/jekyll/jekyll/issues/4831 for some details.
Jekyll 3.0.5 fixes the problem. This commit upgrades to the latest
version of github-pages, thereby also upgrading to Jekyll 3.0.5

Gemfile.lock

 GEM
   remote: http://rubygems.org/
   specs:
-    RedCloth (4.2.9)
     activesupport (4.2.6)
       i18n (~> 0.7)
       json (~> 1.7, >= 1.7.7)
@@ -14,36 +13,32 @@ GEM
       execjs
     coffee-script-source (1.10.0)
     colorator (0.1)
-    ethon (0.8.1)
+    ethon (0.9.0)
       ffi (>= 1.3.0)
-    execjs (2.6.0)
+    execjs (2.7.0)
     faraday (0.9.2)
       multipart-post (>= 1.2, < 3)
-    ffi (1.9.10)
+    ffi (1.9.13)
     gemoji (2.1.0)
-    github-pages (75)
-      RedCloth (= 4.2.9)
+    github-pages (86)
       github-pages-health-check (= 1.1.0)
-      jekyll (= 3.0.4)
+      jekyll (= 3.1.6)
       jekyll-coffeescript (= 1.0.1)
       jekyll-feed (= 0.5.1)
       jekyll-gist (= 1.4.0)
-      jekyll-github-metadata (= 1.11.1)
-      jekyll-mentions (= 1.1.2)
+      jekyll-github-metadata (= 2.0.2)
+      jekyll-mentions (= 1.1.3)
       jekyll-paginate (= 1.1.0)
       jekyll-redirect-from (= 0.10.0)
       jekyll-sass-converter (= 1.3.0)
-      jekyll-seo-tag (= 1.3.3)
+      jekyll-seo-tag (= 2.0.0)
       jekyll-sitemap (= 0.10.0)
-      jekyll-textile-converter (= 0.1.0)
       jemoji (= 0.6.2)
-      kramdown (= 1.10.0)
+      kramdown (= 1.11.1)
       liquid (= 3.0.6)
       listen (= 3.0.6)
       mercenary (~> 0.3)
-      rdiscount (= 2.1.8)
-      redcarpet (= 3.3.3)
-      rouge (= 1.10.1)
+      rouge (= 1.11.1)
       terminal-table (~> 1.4)
     github-pages-health-check (1.1.0)
       addressable (~> 2.3)
@@ -51,11 +46,11 @@ GEM
       octokit (~> 4.0)
       public_suffix (~> 1.4)
       typhoeus (~> 0.7)
-    html-pipeline (2.4.0)
+    html-pipeline (2.4.1)
       activesupport (>= 2, < 5)
       nokogiri (>= 1.4)
     i18n (0.7.0)
-    jekyll (3.0.4)
+    jekyll (3.1.6)
       colorator (~> 0.1)
       jekyll-sass-converter (~> 1.0)
       jekyll-watch (~> 1.1)
@@ -69,9 +64,10 @@ GEM
     jekyll-feed (0.5.1)
     jekyll-gist (1.4.0)
       octokit (~> 4.2)
-    jekyll-github-metadata (1.11.1)
+    jekyll-github-metadata (2.0.2)
+      jekyll (~> 3.1)
       octokit (~> 4.0)
-    jekyll-mentions (1.1.2)
+    jekyll-mentions (1.1.3)
       html-pipeline (~> 2.3)
       jekyll (~> 3.0)
     jekyll-paginate (1.1.0)
@@ -79,45 +75,43 @@ GEM
       jekyll (>= 2.0)
     jekyll-sass-converter (1.3.0)
       sass (~> 3.2)
-    jekyll-seo-tag (1.3.3)
-      jekyll (~> 3.0)
+    jekyll-seo-tag (2.0.0)
+      jekyll (~> 3.1)
     jekyll-sitemap (0.10.0)
-    jekyll-textile-converter (0.1.0)
-      RedCloth (~> 4.0)
-    jekyll-watch (1.3.1)
-      listen (~> 3.0)
+    jekyll-watch (1.4.0)
+      listen (~> 3.0, < 3.1)
     jemoji (0.6.2)
       gemoji (~> 2.0)
       html-pipeline (~> 2.2)
       jekyll (>= 3.0)
     json (1.8.3)
-    kramdown (1.10.0)
+    kramdown (1.11.1)
     liquid (3.0.6)
     listen (3.0.6)
       rb-fsevent (>= 0.9.3)
       rb-inotify (>= 0.9.7)
     mercenary (0.3.6)
-    mini_portile2 (2.0.0)
-    minitest (5.8.4)
+    mini_portile2 (2.1.0)
+    minitest (5.9.0)
     multipart-post (2.0.0)
     net-dns (0.8.0)
-    nokogiri (1.6.7.2)
-      mini_portile2 (~> 2.0.0.rc2)
+    nokogiri (1.6.8)
+      mini_portile2 (~> 2.1.0)
+      pkg-config (~> 1.1.7)
     octokit (4.3.0)
       sawyer (~> 0.7.0, >= 0.5.3)
+    pkg-config (1.1.7)
     public_suffix (1.5.3)
     rb-fsevent (0.9.7)
     rb-inotify (0.9.7)
       ffi (>= 0.5.0)
-    rdiscount (2.1.8)
-    redcarpet (3.3.3)
-    rouge (1.10.1)
+    rouge (1.11.1)
     safe_yaml (1.0.4)
     sass (3.4.22)
     sawyer (0.7.0)
       addressable (>= 2.3.5, < 2.5)
       faraday (~> 0.8, < 0.10)
-    terminal-table (1.5.2)
+    terminal-table (1.6.0)
     thread_safe (0.3.5)
     typhoeus (0.8.0)
       ethon (>= 0.8.0)
@@ -132,4 +126,4 @@ DEPENDENCIES
   rouge
 
 BUNDLED WITH
-   1.11.2
+   1.12.5