Fix gem name in config

[KevinTCoughlin]

First off, thanks for this awesome gem and your work w/ Jekyll 😄. Took me a few to realize this is why I couldn't get the gem to run while profiling via jekyll build --profile.

[KevinTCoughlin]

🤔

Failures:
  1) JekyllIncludeCache with an empty cache initializess the cache
     Failure/Error: before { described_class.remove_instance_variable("@cache") }
     
     NameError:
       instance variable @cache not defined
     # ./spec/jekyll-include-tag_spec.rb:3:in `remove_instance_variable'
     # ./spec/jekyll-include-tag_spec.rb:3:in `block (3 levels) in <top (required)>'
Finished in 0.05425 seconds (files took 0.67191 seconds to load)
11 examples, 1 failure
Failed examples:
rspec ./spec/jekyll-include-tag_spec.rb:5 # JekyllIncludeCache with an empty cache initializess the cache

[benbalter]

Thanks for this @KevinTCoughlin! That failing test is likely an order dependency and unrelated.