Allow json gem versions at least 1.7 but less than 2.0

@MichaelAllenClark, here's a simple commit which relaxes the JSON gem version requirement to allow 1.7.x, 1.8.x, and so on, up to any version less than 2.0.

I didn't see any contributor guidelines, but I did run 'rspec spec', and all the specs passed, except for this one:
  1) FuelSDK::Targeting should respond to #determine_stack
     Failure/Error: it { should respond_to(:determine_stack) }
       expected #<#<Class:0x007fc48ea69820>:0x007fc48ea697d0> to respond to :determine_stack

It looks like that's an existing failure which seems to happen because FuelSDK::Targeting.determine_stack is protected - when I got rid of the protected declaration the spec passed. I'm not sure how you want to resolve that.