Update Travis key

Tests run properly but it won't deploy; Travis isn't providing the encrypted key (even when building `master`). ([The build](https://travis-ci.org/sympy/sympy_gamma/builds/27322380) should start with `export OAUTH_REFRESH_TOKEN=[secure]`.) I think it's because I generated the key encrypted for my fork instead of the main repo, so I have regenerated the key for `sympy/sympy_gamma`. I have also updated the README to explain this.