Only require Mock on Python 3.4 and older. Use the builtin one elsewhere.

Mock 2.0 is using pbr which is hostile to environments without network access.
It's not required on Python 3.5+ so I made it possible to use `unittest.mock`
on this version.

Updated tox.ini to reflect this. Tested with 2.7.11, 3.3.6, 3.4.5, and 3.5.2 on
macOS 10.12.1 with tox and pyenv.

tests/integration/test_checker.py

@@ -1,5 +1,8 @@
 """Integration tests for the checker submodule."""
-import mock
+try:
+    import mock
+except ImportError:
+    from unittest import mock
 import pytest
 
 from flake8 import checker