Add Gemfile dependency for require_relative on mri18. This is a dependency of...

Add Gemfile dependency for require_relative on mri18. This is a dependency of linecache which is a dependency of ruby-debug. However, the latest release of linecache (0.45) omits the dependency from its gemspec, causing the build to fail. I have emailed the maintainer so we should remove this when it's fixed upstream.

Gemfile

@@ -26,6 +26,9 @@ gem "memcache-client", ">= 1.8.5"
 
 platforms :mri_18 do
   gem "system_timer"
+  # ruby-debug requires linecache which depends on require_relative but doesn't explicitly
+  # declare this in its gemspec
+  gem "require_relative"
   gem "ruby-debug", ">= 0.10.3"
   gem "json"
 end