Build management tooling

This is the build scripts that I recently revamped in rspec-mocks.  I want to have a standard way to manage them across all repos, so this is a first pass at that.  The idea is that we'll no longer update the build scripts in the individual repos by hand; instead you update them here in rspec-dev, and run the rake task which copies them to each repo, commits, pushes and creates a PR with the change.  That way we can see it go green in each repo before the build changes are merged.

Two open issues:

* ~~I used this to create PRs for the other repos (rspec/rspec-core#1259, rspec/rspec-mocks#352, rspec/rspec-expectations#433, rspec/rspec-support#29) but the builds are failing in all of them except rspec-mocks with a permissions error on the `clone_all_rspec_repos` script.~~

* Do we want to use this for rspec-rails?  Also, do we want the builds of the other repos to clone rspec-rails and run its spec suite (like we do for all the others)?  I'm not very familiar with the rspec-rails build but I know it's different so I left it out for now.  @alindeman -- this'll really be your call here so let us know what you think.

/cc @JonRowe @samphippen @xaviershay @soulcutter