Fix "undefined method configure bug (#21)

* Fix "undefined method `configure` bug

Due to rake eagerly loading the jsonapi-rspec gem before the RSpec:Core module,  configure isn't available when rspec.rb tries to use it.  Requiring rspec/core resolves this issue.

* Add rspec-core to the dependency list

Author: sa73917

jsonapi-rspec.gemspec

@@ -13,6 +13,7 @@ Gem::Specification.new do |spec|
   spec.files         = Dir['README.md', 'lib/**/*']
   spec.require_path  = 'lib'
 
+  spec.add_dependency 'rspec-core'
   spec.add_dependency 'rspec-expectations'
 
   spec.add_development_dependency 'rake'

lib/jsonapi/rspec.rb

@@ -1,5 +1,6 @@
 require 'json'
 require 'rspec/matchers'
+require 'rspec/core'
 require 'jsonapi/rspec/id'
 require 'jsonapi/rspec/type'
 require 'jsonapi/rspec/attributes'