Updates to Travis-CI configuration to fix Ruby 1.8.7 builds

Change adds workaround for failing build on Ruby 1.8.7:

* https://github.com/bundler/bundler/issues/2784
* https://github.com/rspec/rspec-expectations/commit/d2e3d97e5f0f1e23061f26376d2a752292c2a5d2