Stop snapshot publishing to sonatype during travis CI for anything but the master branch

[nh13]

I notice that when we push new commits to a PR, travis CI still pushes a snapshot to sonatype for one of the two builds. Using TRAVIS_BRANCH and TRAVIS_PULL_REQUEST may not be sufficient. See: https://docs.travis-ci.com/user/environment-variables/

[nh13]

Done.