Updates rest-client version from '~> 1.6.7' to '~> 1.7.2'.

I need to bump the version of the `rest-client` dependency from `'~> 1.6.7'` to  `'~> 1.7.2'` because it is currently causing conflicts with my integration test runner gem.

There aren't any tests that I can see, but I have used this locally and it works.