Add "require 'representable/hash'"

Without including the base "representable" representable/json will error
out.

versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/representable-1.8.3/lib/representable/json.rb:6:in
`<module:JSON>': uninitialized constant Hash::ClassMethods (NameError)

This patch will include the hash module and correct it.
Merge pull request #95 from CodingZeal/master

Include 'representable/hash' to fix a breaking change from a recent patch-level upgrade