Add gravity forms to dokku env

[MlKilderkin]

What does this do/fix?

Add gravity forms to dokku env

QA

Links to relevant issues

• https://moderntribe.atlassian.net/browse/UCSC-143

Tests

Does this have tests?

• [ ] Yes
• [ ] No, this doesn't need tests because...
• [ ] No, I need help figuring out how to write the tests.