Set git branch name from environment if available

When using wercker, the git tree is checked out by sha so running `git rev-parse --abbrev-ref HEAD` simply returns `HEAD`. This is not so helpful in coveralls.

This patch enables the possibility of using the `GIT_BRANCH` environment variable to get the branch name.

When using wercker, the branch is set to the `WERCKER_GIT_BRANCH` environment variable so something like this is still necessary in the `wercker.yml`:

```yaml
build:
    steps:
        ...

        # Test the project
        - script:
            name: go test
            code: |
                export GIT_BRANCH=$WERCKER_GIT_BRANCH
                goveralls -v -service wercker -repotoken $COVERALLS_REPO_TOKEN
```