Update Gemfile.lock for version 5.2.0

I get the following error with commit a26ba07 (Version 5.2.0)

```
% bundle install --path=vendor/bundle --deployment
Fetching gem metadata from http://rubygems.org/.........
Fetching additional metadata from http://rubygems.org/..
Could not find gitlab_git-5.1.0 in any of the sources
```

This could be fixed by updating Gemfile.lock (or `bundle install --no-deployment`).

just for your information:

```
% ruby --version
ruby 2.1.0p0 (2013-12-25 revision 44422) [x86_64-darwin13.0]
% bundle --version
Bundler version 1.5.3
```