Remove 'require "rake"' from gemspec

[parkr]

In Ruby 2.3.1, rake is not installed automatically. Therefore, loading it is impossible... Error message:

+ rbenv exec bundle install --local --path .bundle --no-cache --without development test

There was a LoadError while loading hoosegow.gemspec:
cannot load such file -- rake from
  /hoosegow/hoosegow.gemspec:1:in `<main>'

Does it try to require a relative path? That's been removed in Ruby 1.9.

/cc @spraints @mastahyeti for review

[spraints]

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