Add setuptools update to .travis.yml

A bug in mocks `setup.py` means that setuptools cannot find the six module. Updating setuptools satisfies this dependency.

pypa/setuptools#1002 (comment)
jaraco (setuptools contributor) 'You can't use setup_requires to indicate setuptools'

https:/testing-cabal/mock/blob/master/setup.py#L5
mock/setup.py
```
import setuptools

setuptools.setup(
    setup_requires=['pbr>=1.3', 'setuptools>=17.1'],
    pbr=True)
```

.travis.yml

@@ -3,6 +3,7 @@ python:
  - "2.7"
  - "3.4"
 install:
+ - pip install -U setuptools
  - pip install --no-use-wheel -r requirements_for_test.txt
 script:
  - make test