Dependabot-core: Support custom named Gemfiles

[tjwallace]

Our overcommit setup uses a separate gemfile (.overcommit_gemfile.rb). It would be great if dependabot allowed the name of the gemfile to be configured. You can use the BUNDLE_GEMFILE environment variable.

[lloydpick]

We also have a similar issue where we have multiple gemfiles, the standard in Gemfile and another one in Gemfile.deploy. It would be great if dependabot could check both.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs within seven days. Thank you for your contributions.

[tjwallace]

This would still be really helpful to us