Update/fix dependencies to remove warnings

[kattrali]

Enabling warnings during the spec phase turns up a few warnings from testing libraries. Look into a way to mitigate them:

/gem_home/webmock-1.22.3/lib/webmock/request_body_diff.rb:20: warning: private attribute?
/gem_home/webmock-1.22.3/lib/webmock/request_body_diff.rb:20: warning: private attribute?
/gem_home/webmock-1.22.3/lib/webmock/request_execution_verifier.rb:41: warning: assigned but unused variable - is_negated
/gem_home/webmock-1.22.3/lib/webmock/request_execution_verifier.rb:45: warning: assigned but unused variable - is_negated
/gem_home/httpclient-2.6.0.1/lib/httpclient.rb:1083: warning: assigned but unused variable - e
/gem_home/httpclient-2.6.0.1/lib/httpclient/auth.rb:538: warning: assigned but unused variable - domain_uri
/gem_home/httpclient-2.6.0.1/lib/httpclient/webagent-cookie.rb:413: warning: private attribute?

Split from #254.