Fix Segfault on Ruby 2.1.0

Inspired by this fix: https://github.com/rollbar/rollbar-gem/commit/ae36b448b5b3cf16aec64c3db24ccafc93371638

We were getting segfaults running in production under 2.1.0.

You can reproduce the segfault by running the specs for this gem under 2.1.0. 

This fixes it under 2.1.0, and specs still pass for 2.0.0 and 1.9.3. This also adds 2.1.0 to the .travis.yml.