split travis tests into groups

- run quick tests separately
- split the db tests into 3 groups (by app_label, alphabetically)

for a total for 4 targets
tweak TRAVIS_TEST_GROUPS in hopes of balancing out times
tweak TRAVIS_TEST_GROUPS in hopes of balancing out times
clean up TRAVIS_TEST_GROUPS setting
Merge pull request #3371 from dimagi/split-up-travis-tests

split travis tests into groups