tox: update swift version

There some features we can make use of in Swift 2.0:

 - HEAD requests support if-modified and if-unmodified headers.
 - PUT requests support if-none-match headers.  It can be used for copy
   requests.

Let's hack with the latest Swift!

Change-Id: I29c6c42513b4387919f954c478cdd8f35280fd83