Duplicate 2.7.1 as 2.6.11

This is a hack to sneak into Gemfiles that ask for gitlab-grit ~> 2.6.x.
The changes relative to 2.6.10 justify making this a patch release.

Gemfile.lock

 PATH
   remote: .
   specs:
-    gitlab-grit (2.7.1)
+    gitlab-grit (2.6.11)
       charlock_holmes (~> 0.6)
       diff-lcs (~> 1.1)
       mime-types (~> 1.15)

History.txt

-== 2.7.1
+== 2.6.11
   * Suppress 'unkown header' warnings
   * Add process.out to CommandFailed error 

VERSION

-2.7.1
+2.6.11