Upgrade letter_opener_web to support Rails 5.1

Before this commit using `/rails/letter_opener` in `development`
environment failed with:

    undefined method `before_filter'

See https://github.com/fgrehm/letter_opener_web/issues/68

This commit upgrades `letter_opener_web` to 1.3.4 so Rails 5.1 is
supported.

Gemfile

@@ -309,7 +309,7 @@ group :development do
   gem 'foreman', '~> 0.84.0'
   gem 'brakeman', '~> 4.2', require: false
 
-  gem 'letter_opener_web', '~> 1.3.0'
+  gem 'letter_opener_web', '~> 1.3.4'
   gem 'rblineprof', '~> 0.3.6', platform: :mri, require: false
 
   # Better errors handler

Gemfile.lock

@@ -444,9 +444,9 @@ GEM
       rest-client (~> 2.0)
     launchy (2.4.3)
       addressable (~> 2.3)
-    letter_opener (1.4.1)
+    letter_opener (1.7.0)
       launchy (~> 2.2)
-    letter_opener_web (1.3.0)
+    letter_opener_web (1.3.4)
       actionmailer (>= 3.2)
       letter_opener (~> 1.0)
       railties (>= 3.2)
@@ -1093,7 +1093,7 @@ DEPENDENCIES
   kaminari (~> 1.0)
   knapsack (~> 1.17)
   kubeclient (~> 4.2.2)
-  letter_opener_web (~> 1.3.0)
+  letter_opener_web (~> 1.3.4)
   license_finder (~> 5.4)
   licensee (~> 8.9)
   lograge (~> 0.5)

changelogs/unreleased/pl-upgrade-letter_opener_web.yml

+---
+title: Upgrade letter_opener_web to support Rails 5.1
+merge_request: 27829
+author:
+type: fixed