Fix byebug error

[misaka]

Not certain why but in certain circumstances ruby seems to lose it's
link to the readline dylib causing byebug to raise errors when being
required, possibly as a result of a 'brew update'. The fix in this
commit is mentioned here:

deivid-rodriguez/byebug#289

The alternative seems to be to recompile ruby.

[misaka]

To help others, the error I get when trying to run anything in this project:

    Sorry, you can't use byebug without Readline. To solve this, you need to
    rebuild Ruby with Readline support. If using Ubuntu, try `sudo apt-get
    install libreadline-dev` and then reinstall your Ruby.
/Users/michaelgorodnitzky/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/byebug-9.0.5/lib/byebug/history.rb:2:in `require': dlopen(/Users/michaelgorodnitzky/.rbenv/versions/2.3.1/lib/ruby/2.3.0/x86_64-darwin15/readline.bundle, 9): Library not loaded: /usr/local/opt/readline/lib/libreadline.6.dylib (LoadError)
  Referenced from: /Users/michaelgorodnitzky/.rbenv/versions/2.3.1/lib/ruby/2.3.0/x86_64-darwin15/readline.bundle
  Reason: image not found - /Users/michaelgorodnitzky/.rbenv/versions/2.3.1/lib/ruby/2.3.0/x86_64-darwin15/readline.bundle