Ignore release_xxx branches in travis
Ignore release_xxx branches in travis

We've already tagged them as good in our CI environment; it seems
wasteful to repeat the build in travis.
alphagov/trade-tariff-backend,bitzesty/trade-tariff-backend,bitzesty/trade-tariff-backend,bitzesty/trade-tariff-backend,alphagov/trade-tariff-backend,leftees/trade-tariff-backend,leftees/trade-tariff-backend,leftees/trade-tariff-backend,alphagov/trade-tariff-backend