Fix tests, add py35

- Add Python 3.5 support on travis

- pytest-cache has been merged into pytest [0] and is no longer
  separately maintained, so pytest-cache-related requirements are being
  updated.

- Flask-Mongoengine latest stable is broken on Python 2.6 (but fixed in
  master [1]), so its version is being restricted for now.

[0] http://pytest.org/latest/changelog.html#id3 [1]
https://github.com/MongoEngine/flask-mongoengine/pull/174

setup.py

@@ -32,7 +32,7 @@ class PyTest(TestCommand):
             '--cov-report', 'term-missing',
             '--pep8',
             '--flakes',
-            '--clearcache'
+            '--cache-clear'
         ]
         self.test_suite = True