Fix macOS build

- Ensure python is available https://github.com/travis-ci/travis-ci/issues/2312
- Only pip install --user on linux, macOS is in virtualenv
- Set PATH in a os-agnostic manner https://github.com/huonw/travis-cargo/pull/71

.travis.yml

@@ -20,8 +20,15 @@ addons:
 
 before_script:
 - |
-  pip install 'travis-cargo<0.2' --user &&
-  export PATH=$HOME/.local/bin:/usr/local/bin:$PATH
+  if [ $TRAVIS_OS_NAME = 'osx' ]; then
+    brew install python3
+    virtualenv env -p python3
+    source env/bin/activate
+    pip install 'travis-cargo<0.2'
+  else
+    pip install 'travis-cargo<0.2' --user
+  fi
+- export PATH="$(python -m site --user-base)/bin:$PATH"
 
 script:
 - |