Bump Haml to 4.0.7 to speed up textarea rendering

Closes #1987

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 7.14.0 (unreleased)
+  - Bump Haml to 4.0.7 to speed up textarea rendering (Stan Hu)
   - Show incompatible projects in Bitbucket import status (Stan Hu)
   - Fix coloring of diffs on MR Discussion-tab (Gert Goet)
   - Fix "Network" and "Graphs" pages for branches with encoded slashes (Stan Hu)

Gemfile.lock

@@ -307,7 +307,7 @@ GEM
     grape-entity (0.4.2)
       activesupport
       multi_json (>= 1.3.2)
-    haml (4.0.5)
+    haml (4.0.7)
       tilt
     haml-rails (0.5.3)
       actionpack (>= 4.0.1)