Fix uncaught LoadError requiring pygments.rb

[davidcelis]

If ghi was installed via Homebrew before pygments.rb was added as a
dependency, the check for pygments being installed never gets run. It
looks like there was intention to rescue that require statement but
because LoadError doesn't inherit from StandardError, the exception
was uncaught and ghi would silently exit. This only seems to happen
currently on issues that include fenced code blocks, but I provided
reproducible evidence in #281. Merging this will fix #281, but I have
two concerns about the fact that pygments.rb was included as a
dependency in the first place:

1. Homebrew doesn't install Ruby gems for you, so this has to be done
   manually. Attempting to brew install ghi without first having run
   gem install pygments.rb just outputs an error and doesn't finish.
2. Most Ruby developers install several versions of Ruby using a version
   manager like rvm, rbenv, or chruby. You can gem install pygments.rb
   on one version of Ruby (or sudo gem install pygments.rb on the
   system version of Ruby, which is a whole other problem) but as soon
   as you try to use ghi after switching Rubies, you have to remember
   to install pygments.rb. This isn't great.

I see two solutions here. We can either no longer offer a Homebrew
formula, instead urging users to just install via gem install ghi or
we can remove pygments.rb as a dependency. Personally I think ghi
should try to remain free of dependencies for this reason, but I leave
it in your hands.

[stephencelis]

Nice catch!