Change gollum repo to use https:// instead of git://

A lot of people might have firewalls blocking the git:// protocol. Using https:// makes this easier and matches the other git repositories in the Gemfile.

Gemfile

@@ -104,7 +104,7 @@ gem 'settingslogic'
 # github-linquist needs pygments 0.4.2 but Gollum 2.4.11
 # requires pygments 0.3.2. The latest master Gollum has been updated
 # to use pygments 0.4.2. Change this after next Gollum release.
-gem "gollum", "~> 2.4.0", git: "git://github.com/gollum/gollum.git", ref: "5dcd3c8c8f"
+gem "gollum", "~> 2.4.0", git: "https://github.com/gollum/gollum.git", ref: "5dcd3c8c8f"
 
 # Misc
 gem "foreman"