Add build_doc.sh script to .travis.yml

I still need to add the encrypted token so that it actually works
Add encrypted token for Travis
build_doc.sh will be one level up
Debugging
Hopefully fix build_doc calling in .travis.yml
Print a message in build_doc in a pull request
Rename build_doc.sh to update-gh-pages.sh
Merge pull request #22 from asmeurer/travis-build-docs

Add build_doc.sh script to .travis.yml