Restrict certain gems on certain versions of ruby

Created by: robotdana

As rspec gitignores its Gemfile.lock it can mean new prs can fail with unrelated changes because bundler will get the most recent versions of gems which might have new and exciting incompatibilities. (as i found with https://github.com/rspec/rspec-core/pull/2749)

Also because the travis agents cache these bundles and don't force an update it can sometimes just work