[Travis] Change test matrix to test only once per php version and db

Moved the timezones to be random per build instead of being in the
matrix, also reduce number of builds so we cover each php version
once and across them a mix of mysql and postgres.

Motivation is to make travis run faster (seems to take it from 40 to 20 minutes), easier to maintain the travis.yml and make space for 5.6 and potentially also hhvm.