Upgrade to listen 3.0.5, re-enable tests

In listen 3.0.4 and below, the `#stop` method on the notification
backends are a no-op, meaning that we are leaking them per test. This
ended up triggering another bug in Ruby that causes our builds to fail
randomly on CI.

listen bug: https://github.com/guard/listen/issues/353
ruby bug: https://bugs.ruby-lang.org/issues/11744

Gemfile

@@ -45,7 +45,7 @@ end
 
 # Active Support.
 gem 'dalli', '>= 2.2.1'
-gem 'listen', '~> 3.0.4'
+gem 'listen', '~> 3.0.5'
 
 # Active Job.
 group :job do

Gemfile.lock

@@ -222,7 +222,7 @@ GEM
     kindlerb (0.1.1)
       mustache
       nokogiri
-    listen (3.0.4)
+    listen (3.0.5)
       rb-fsevent (>= 0.9.3)
       rb-inotify (>= 0.9)
     loofah (2.0.3)
@@ -342,7 +342,7 @@ DEPENDENCIES
   jquery-rails!
   json
   kindlerb (= 0.1.1)
-  listen (~> 3.0.4)
+  listen (~> 3.0.5)
   mail!
   minitest (< 5.3.4)
   mocha (~> 0.14)

activesupport/test/file_evented_update_checker_test.rb

@@ -5,11 +5,6 @@ require 'file_update_checker_shared_tests'
 class FileEventedUpdateCheckerTest < ActiveSupport::TestCase
   include FileUpdateCheckerSharedTests
 
-  def setup
-    skip if ENV['LISTEN'] == '0'
-    super
-  end
-
   def new_checker(files = [], dirs = {}, &block)
     ActiveSupport::FileEventedUpdateChecker.new(files, dirs, &block).tap do
       wait

ci/travis.rb

@@ -111,14 +111,8 @@ class Build
   end
 
   def env
-    if activesupport? && !isolated?
-      # There is a known issue with the listen tests that casuses files to be
-      # incorrectly GC'ed even when they are still in-use. The current is to
-      # only run them in isolation to avoid randomly failing our test suite.
-      { 'LISTEN' => '0' }
-    else
-      {}
-    end
+    # Provide extra ENV variables for the build here
+    {}
   end
 
   def run_bug_report_templates