Fix uploading Doxygen documentation to GH pages from Travis CI

Since the merge of #60, uploading the Doxygen documentation from Travis CI to GitHub pages has been failing due to invalid credential setup.

Instead of the original workaround, this patch changes the authentication to directly setup the `.git-credentials` file for `github.com`, to be used by the `git-credential-store` helper from Git.

Works on my fork's instance at
 * https://travis-ci.org/pah/rapidjson
 * https://pah.github.io/rapidjson