Makes Travis do `docker push` after builds and tests
Makes Travis do `docker push` after builds and tests

- master image is updated with a successful build after a push to master
  branch
- other branches and pull requests will have their images pushed after
  tests

HazyResearch/deepdive,HazyResearch/deepdive,sky-xu/deepdive,shahin/deepdive,shahin/deepdive,HazyResearch/deepdive,HazyResearch/deepdive,sky-xu/deepdive