Replace $GITHUB_TOKEN secret ENV var in travis-ci

[svenlaater]

What is the problem?

This repo uses $GITHUB_TOKEN secret ENV variable in travis ci to upload binaries to github releases. It is tied to my personal github account and might not be ideal.

What do you expect to see / happen?

Replace the $GITHUB_TOKEN in travis ci with something more specific to PoT. The only required scope is "repo > public_repo".

You can trigger a new build in travis-ci from master branch to make sure that it works.