Add .travis.yml

After @flarnie fixed support for Ruby 1.8.7, I want to make sure that
this keeps working. I'm therefore adding a Travis integration, where I
plan to run rspec tests for 1.8.7, 1.9.3 and 2.1.2.
Install rspec as part of Travis run

Since we're not using a bundler to manage dependencies, we need to tell
Travis how to get started before running `rspec`.
Switch from Ruby 2.1.2 to 2.1.1

According to http://docs.travis-ci.com/user/languages/ruby/, version
2.1.1 is pre-installed on Travis servers, so that should make the test
faster. (It currently takes 50 seconds).
Merge pull request #4 from trotzig/travis-ci

Travis ci