Maintenance: Make 'mini_racer' gem dependency for generating assets optional in scenarios where node.js should get used instead.

Gemfile

@@ -62,8 +62,13 @@ gem 'autoprefixer-rails'
 
 # asset handling - javascript execution for e.g. linux
 gem 'execjs'
-gem 'libv8'
-gem 'mini_racer'
+
+# mini_racer can be omitted on systems where node.js is available via
+#   `bundle install --without mini_racer`.
+group :mini_racer do
+  gem 'libv8'
+  gem 'mini_racer'
+end
 
 # authentication - provider
 gem 'doorkeeper'

Gemfile.lock

@@ -201,7 +201,7 @@ GEM
       multipart-post (>= 1.2, < 3)
     faraday-http-cache (2.0.0)
       faraday (~> 0.8)
-    ffi (1.11.1)
+    ffi (1.15.0)
     ffi-compiler (1.0.1)
       ffi (>= 1.0.0)
       rake