added rspec dependency

[klobuczek]

I had to add the dependency as I was getting errors in some context.
rspec should not only be development dependency of jsonapi_spec_helpers as it does require rspec at runtime even though in test environment only.

Exception attached:

heinrich@berlin:~/mck/org-api$ RAILS_ENV=test bundle exec rake neo4j:deploy[test] --trace
rake aborted!
Bundler::GemRequireError: There was an error while trying to load the gem 'jsonapi_spec_helpers'.
Gem Load Error is: uninitialized constant RSpec::Matchers
Backtrace for gem load error is:
/Users/heinrich/mck/jsonapi_spec_helpers/lib/jsonapi_spec_helpers/matchers.rb:1:in `<top (required)>'
/Users/heinrich/mck/jsonapi_spec_helpers/lib/jsonapi_spec_helpers.rb:3:in `<top (required)>'
/Users/heinrich/.rvm/gems/ruby-2.4.0/gems/bundler-1.15.2/lib/bundler/runtime.rb:82:in `require'

[richmolj]

Thanks @klobuczek!

[richmolj]

0.4.2