Adds travis python install instructions for OSX

.travis.yml

@@ -1,6 +1,5 @@
 os:
   - linux
-  - osx
 language: python
 python:
   - "2.7"
@@ -8,6 +7,17 @@ python:
   - "3.4"
   - "3.5"
   - "3.6"
+matrix:
+  include:
+    - language: generic
+      os: osx
+      python:
+        - "3"
+      before_install:
+        - brew update
+        - brew install python3
+        - virtualenv env -p python3
+        - source env/bin/activate
 # command to install dependencies
 install:
     - "pip install -r tests/requirements.txt"