feat: Allow triggering a release via the contrib/release script

- Requires docker, git, and package_cloud gems
- Requires the PACKAGECLOUD_API_TOKEN environment variable
- Builds both a deb and rpm, pushing both to packagecloud
- Upgrades the golang build environment to 1.7.5
- Combines both build Dockerfiles into a single, generic env
- Fixes issue where internal-functions file for tar plugin was not executable
- Ensures that built plugins have a callback to clean up their source files before packaging
- Adds support for “betafish” releases, which are untested versions of Dokku that are available via a separate packagecloud repository.

plugins/repo/Makefile

@@ -21,3 +21,6 @@ subcommands/purge-cache: **/**/**/purge-cache.go
 
 clean:
 	rm -rf commands subcommands
+
+src-clean:
+	rm -rf .gitignore src vendor