only do coverage when using gonum branch

Travis doesn't allow secure env variables (such as the coveralls api
key) to be used when merging a fork. This change avoids doing coverage
under that circumstance, so that tests still run.
Merge pull request #5 from gonum/only-coveralls-if-branch

As this only impacts travis-ci and no active gonum code, I am forgoing the normal review process and merging.