Bump execjs

Also, remove explicit dependency as it originates from autoprefixer-rails

Gemfile

@@ -118,7 +118,6 @@ gem 'sprockets',        '~> 3.5.2'
 gem 'sass-rails',       '~> 5.0.3'
 gem 'sass',             '~> 3.4.12'
 gem 'autoprefixer-rails'
-gem 'execjs',           '~> 2.4.0'
 gem 'bourbon',          '~> 4.2.0'
 
 gem 'prototype-rails', git: 'https://github.com/rails/prototype-rails.git', branch: '4.2'

Gemfile.lock

@@ -232,7 +232,7 @@ GEM
     erubis (2.7.0)
     eventmachine (1.0.7)
     excon (0.45.3)
-    execjs (2.4.0)
+    execjs (2.6.0)
     factory_girl (4.5.0)
       activesupport (>= 3.0.0)
     factory_girl_rails (4.5.0)
@@ -601,7 +601,6 @@ DEPENDENCIES
   date_validator (~> 0.9.0)
   delayed_job_active_record (~> 4.1.0)
   equivalent-xml (~> 0.5.1)
-  execjs (~> 2.4.0)
   factory_girl (~> 4.5)
   factory_girl_rails (~> 4.5)
   faker