Revert "Remove therubyracer gem (unused)"

This reverts commit acc5fff05d.

Gemfile

@@ -10,6 +10,9 @@ gem 'sass-rails', '~> 5.0'
 # Use Uglifier as compressor for JavaScript assets
 gem 'uglifier', '>= 1.3.0'
 
+# See https://github.com/sstephenson/execjs#readme for more supported runtimes
+gem 'therubyracer', platforms: :ruby
+
 # Use jquery as the JavaScript library
 gem 'jquery-rails'
 # Turbolinks makes following links in your web application faster. Read more: https://github.com/rails/turbolinks

Gemfile.lock

@@ -344,6 +344,7 @@ GEM
     leaflet-markercluster-rails (0.7.0)
       railties (>= 3.1)
     leaflet-rails (0.7.7)
+    libv8 (3.16.14.17)
     listen (3.1.5)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
@@ -460,6 +461,7 @@ GEM
       trollop (~> 2.1)
     rdoc (4.3.0)
     redis (3.3.0)
+    ref (2.0.0)
     request_store (1.3.1)
     responders (2.3.0)
       railties (>= 4.2.0, < 5.1)
@@ -552,6 +554,9 @@ GEM
       httpclient (>= 2.4)
       i18n
       json (>= 1.4.3)
+    therubyracer (0.12.2)
+      libv8 (~> 3.16.14.0)
+      ref
     thor (0.19.4)
     thread_safe (0.3.5)
     tilt (2.0.5)
@@ -670,6 +675,7 @@ DEPENDENCIES
   spring
   spring-commands-rspec
   sqlite3
+  therubyracer
   timecop
   turbolinks (~> 2.5)
   uglifier (>= 1.3.0)