Only deploy to GitHub pages on master.

Only deploy to GitHub pages if [variables are set](http://docs.travis-ci.com/user/ci-environment/#Environment-variables):

- `$TRAVIS_PULL_REQUEST` is false
- `$TRAVIS_BRANCH` is 'master' (or 'travis')
- `$TRAVIS_PYTHON_VERSION` is 3.3 (to only push once)

and only deploy once per build. Also consider adding a `make pages` target to remove some of commands from `.travis.yml`.