Fix tests to work with current webassets==dev

* Remove TestVersionSystemDeprecations test class.
  Webassets 0.7 deprecation warnings are removed at
  miracle2k/webassets@29d393a55bdd1b5bce170e7a6032c0764a15e112.

* Use url_expire option instead of removed expire option
  to preserve test outputs.

* Remove py25_flake08 env from tox.ini and .travis.yml.
  268e83f1167ce45ac5b36048461370e9c5a84f54 has already dropped
  Python 2.5 support