Update rest-client for security fix

[albertsun]

The version of RestClient specified in the gemspec has this security flaw: http://www.osvdb.org/show/osvdb/117461

I believe it should be updated to at least 1.7.3
https://github.com/rest-client/rest-client/blob/master/history.md

[cantino]

I think we need to bump to 1.8.0 to resolve the security issues.