Warn when virtualenv is not installed.
Fixes issue #78 - unicode path comparison

After cloning the repo and running
```
$ python setup.py test

==========================================
FAIL: paver.tests.test_path.test_join_on_unicode_path
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/[... snip ...]/paver/paver/tests/test_path.py", line 24, in test_join_on_unicode_path
    assert expected == os.path.join(paver.path.path('something'), unicode_o)
AssertionError

----------------------------------------------------------------------
Ran 98 tests in 31.354s

FAILED (failures=1, skipped=2)
```
Merge pull request #137 from GitSullied/fix_unicode_path_test

Fixes issue #78 - unicode path comparison
Merge pull request #136 from vbabiy/warn-about-missing-virtualenv

Warn when virtualenv is not installed.
Upgrade six to 1.6.1
Report travis to gitter
Add Dockerfile & info about testing