Replace setting custom CORS headers with rack-cors, set it on /oauth/token endpoint

Gemfile.lock

@@ -157,7 +157,6 @@ GEM
       activerecord (>= 3.1)
     letter_opener (1.4.1)
       launchy (~> 2.2)
-    libv8 (3.16.14.15)
     link_header (0.0.8)
     lograge (0.4.1)
       actionpack (>= 4, < 5.1)
@@ -219,6 +218,7 @@ GEM
     rack (2.0.1)
     rack-attack (5.0.1)
       rack
+    rack-cors (0.4.0)
     rack-protection (1.5.3)
       rack
     rack-test (0.6.3)
@@ -281,7 +281,6 @@ GEM
       redis-store (~> 1.2.0)
     redis-store (1.2.0)
       redis (>= 2.2)
-    ref (2.0.0)
     responders (2.3.0)
       railties (>= 4.2.0, < 5.1)
     rspec (3.5.0)
@@ -349,9 +348,6 @@ GEM
       activesupport (>= 4.0)
       sprockets (>= 3.0.0)
     temple (0.7.7)
-    therubyracer (0.12.2)
-      libv8 (~> 3.16.14.0)
-      ref
     thor (0.19.1)
     thread_safe (0.3.5)
     tilt (2.0.5)
@@ -419,6 +415,7 @@ DEPENDENCIES
   puma
   rabl
   rack-attack
+  rack-cors
   rails (= 5.0.0.1)
   rails_12factor
   rails_autolink
@@ -433,7 +430,6 @@ DEPENDENCIES
   sidekiq
   simple_form
   simplecov
-  therubyracer
   uglifier (>= 1.3.0)
   webmock
   will_paginate