Add travis and lock RSpec version to 2.13.0

Specifies an RSpec version, in line with https://github.com/mixpanel/mixpanel-ruby/pull/40

Note, this will cause a (tiny) merge conflict with https://github.com/mixpanel/mixpanel-ruby/pull/41, but that should be pretty easy to handle!

Specs passing on my fork (https://travis-ci.org/greysteil/mixpanel-ruby/builds/28080873). Instructions for setting up Travis on this repo in https://github.com/mixpanel/mixpanel-ruby/pull/40.