Update less-rails to fix deprecation msg in testing

`rails s` still yield the deprecation warning:
* See issue: metaskills/less-rails#122
* PR pending: metaskills/less-rails#137
* Workaround: `gem 'less-rails', github: 'brendon/less-rails', branch: 'fix-sprockets-loading'

Gemfile

@@ -28,7 +28,7 @@ gem 'devise', '~> 3.5' # Authentication
 # https://samsaffron.com/archive/2015/03/31/debugging-memory-leaks-in-ruby
 # Migration from LESS to SASS is required to drop this dependency
 gem 'therubyracer', '~> 0.12.3', platforms: :ruby
-gem 'less-rails', '~> 2.7'
+gem 'less-rails', '~> 2.8'
 gem 'uglifier', '>= 1.3.0'
 gem 'jquery-rails'
 gem 'coffee-rails', '~> 4.1.1'

Gemfile.lock

@@ -153,7 +153,7 @@ GEM
       addressable (~> 2.3)
     less (2.6.0)
       commonjs (~> 0.2.7)
-    less-rails (2.7.1)
+    less-rails (2.8.0)
       actionpack (>= 4.0)
       less (~> 2.6.0)
       sprockets (> 2, < 4)
@@ -296,7 +296,7 @@ GEM
       rack (~> 1.0)
     thor (0.19.4)
     thread_safe (0.3.6)
-    tilt (2.0.2)
+    tilt (2.0.7)
     turbolinks (2.5.3)
       coffee-rails
     tzinfo (1.2.3)
@@ -344,7 +344,7 @@ DEPENDENCIES
   jquery-rails
   jquery-turbolinks
   launchy
-  less-rails (~> 2.7)
+  less-rails (~> 2.8)
   meta_request
   pg
   pry