require json at spec_helper to fix #532

[gnomex]

Fixes, details at #532

Failures:

  1) DashTestDefaultProc to_json behaves correctly with default proc
     Failure/Error: to_hash.to_json(*args)
     
     NoMethodError:
       undefined method `to_json' for {:fields=>[]}:Hash
       Did you mean?  to_s
     # ./lib/hashie/hash.rb:43:in `to_json'
     # ./spec/hashie/dash_spec.rb:61:in `block (2 levels) in <top (required)>'

Finished in 0.18124 seconds (files took 0.31668 seconds to load)
673 examples, 1 failure

Failed examples:

rspec ./spec/hashie/dash_spec.rb:59 # DashTestDefaultProc to_json behaves correctly with default proc

The solution was appointed by @dblock by adding require 'json' to spec_helper.rb