Support finding lockfile based on BUNDLE_GEMFILE

I have some libraries using the `gemfile` option in travis.yml to run against multiple different gemfiles. These projects ship without a top-level Gemfile.lock; instead, bundle will generate individual lockfiles for each entry in the `gemfile` array.

Using the option sets the `BUNDLE_GEMFILE` environment variable, so if that is set, you can simply append ".lock" to locate the lockfile, rather than assuming it will be in `./Gemfile.lock`.