Add fetching of current branch from git to Configurations class

https://github.com/fs/codeclimate_ci/issues/30
@timurvafin, instead of 
```ruby
Git.open(Dir.pwd).current_branch
```
We have to write
```ruby
repo = Rugged::Repository.new(Dir.pwd)
repo.head.name.sub(/^refs\/heads\//, '')
```