Revert "Switch to PostgreSQL on Travis CI [ci skip]"
Revert "Switch to PostgreSQL on Travis CI [ci skip]"

This reverts commit 5d5d51c98b308ba044e432d95ccfc0b98472626e.

See also: https://github.com/travis-ci/travis-ci/issues/4264

mtsar/mtsar,dustalov/mtsar,dustalov/mtsar,mtsar/mtsar,dustalov/mtsar,mtsar/mtsar