Fix time-based tests for timezone changes (again).

Need to use Time.now so that times don't get mangled in different timezones.
Add the test-unit gem so that we can test in ruby 2.2.
Tie down the webmock version number more carefully.

They broke their API in a minor version update :-(
Cache the gems used in the tests in Travis.
Use a fixed libxml-ruby so it builds on ruby 2.2.