Only deploy tagged releases to PyPI.

This way we don't have to merge pre-release versions into `master` to deploy betas to PyPI.

Author: todofixthis

.travis.yml

@@ -8,8 +8,8 @@ install: pip install .
 script: nosetests
 deploy:
   on:
-    branch: master
     python: '3.6'
+    tags: true
   provider: pypi
   distribution: 'bdist_wheel sdist'
   skip_upload_docs: true