Add a workaround for Mac OS Python 3.5

The problem is a TLS protocol mismatch (PyPI wants TLS 1.2 or newer,
Python 3.5 doesn't support it, pip takes great pains to use alternative
http/tls implementations to make everything work somehow).

.travis.yml

@@ -74,6 +74,9 @@ before_install:
 install:
   - pip install -U pip setuptools
   - pip install -U coveralls coverage
+  # pip install zope.proxy here because because setup_requires will use
+  # easy_install, which fails on Python 3.5 on Mac OS
+  - pip install -U zope.proxy
   - pip install -U -e .[test,docs]
 
 script: