Improve frozen-string-literals compatibility.

[pat]

A few simple changes to allow this gem to be used when RUBYOPT="--enable-frozen-string-literal" is set in MRI 2.4+.

However, it's worth noting that Bacon isn't frozen-string-literal friendly, so the test suite can't be run with this flag. I've addressed this in a PR, but there hasn't been a gem release of Bacon for quite some time (as noted in #45) - so it's worth considering changing to a git repo reference in the Gemfile? This way, you'll be able to add the RUBYOPT flag for 2.4+ versions of Ruby in your test suite and avoid regression errors :)