Pin pip version compatible with Python 3.2 (3rd try)

See problems in issue #84

.travis.yml

@@ -2,7 +2,7 @@ language: python
 python: "3.5"
 install:
   # continue to support Python 3.2 (see issue #84)
-  - pip install --upgrade "pip<8.0.0"
+  - pip install pip==7.1.2
   - pip install coveralls tox
 script:
   - tox

tox.ini

@@ -9,6 +9,7 @@ commands =
     coverage run setup.py test
     sh -c 'coveralls | true'
 deps =
+    pip==7.1.2
     coverage==3.7.1
     coveralls
     django17: Django>=1.7,<1.8