Fix regression caused by using newer version of Grape

This commit fixes the regression caused by using a newer version of Grape and Grape entity.  This solves Issue 4023.

Gemfile

@@ -41,8 +41,8 @@ gem "gitlab-gollum-lib", "~> 1.0.0", require: 'gollum-lib'
 gem "github-linguist", require: "linguist"
 # API
-gem "grape"
-gem "grape-entity"
+gem "grape", "~> 0.3.1"
+gem "grape-entity", "~> 0.2.0"
 # Format dates and times
 # based on human-friendly examples

Gemfile.lock

@@ -173,7 +173,7 @@ GEM
     gon (4.1.0)
       actionpack (>= 2.3.0)
       json
-    grape (0.4.1)
+    grape (0.3.1)
       activesupport
       builder
       hashie (>= 1.2.0)
@@ -183,7 +183,7 @@ GEM
       rack-accept
       rack-mount
       virtus
-    grape-entity (0.3.0)
+    grape-entity (0.2.0)
       activesupport
       multi_json (>= 1.3.2)
     growl (1.0.3)