fix Travis breakage

Travis started failing with:

  uninitialized constant RSpec::Matchers::BuiltIn::RaiseError::MatchAliases (NameError)

due to bundler installing rspec-expectations-2.99 which is a
pre-release:

  https://github.com/rubygems/rubygems/issues/853
Merge pull request #114 from aspiers/fix-travis

fix Travis breakage