Update gitlab-grit to 2.6.11

From the gitlab-grit changelog:
  * Suppress 'unkown header' warnings
  * Add process.out to CommandFailed error

Gemfile.lock

@@ -168,7 +168,7 @@ GEM
       multi_json
     gitlab-grack (2.0.0.pre)
       rack (~> 1.5.1)
-    gitlab-grit (2.6.10)
+    gitlab-grit (2.6.11)
       charlock_holmes (~> 0.6)
       diff-lcs (~> 1.1)
       mime-types (~> 1.15)