Pin/set dependencies for tests

- Pin Sphinx to 1.2.3
  tox installs --pre versions and there's a weird error I don't know how to fix
  in 1.3b2:

  <autodoc>:None: WARNING: more than one target found for cross-reference
  u'BoundLogger': structlog.BoundLogger, structlog.stdlib.BoundLogger,
  structlog.twisted.BoundLogger

- Require a recent-ish version of freezegun

  Ran into
  spulec/freezegun@15c7cc3

setup.py

@@ -76,7 +76,7 @@ def run_tests(self):
             'Topic :: Software Development :: Libraries :: Python Modules',
         ],
         tests_require=[
-            "freezegun",
+            "freezegun>=0.2.8",
             "pretend",
             "pytest-cov",
             "twisted",

tox.ini

@@ -28,7 +28,7 @@ setenv =
     PYTHONHASHSEED = 0
 deps =
     releases
-    sphinx
+    sphinx==1.2.3
     twisted
 commands =
     sphinx-build -W -b html -d {envtmpdir}/doctrees docs docs/_build/html