Enforce knex version to 0.5.x

Latest `knex` (0.6.x) somehow doesn't play well so we need to make sure we're using 0.5.x I didn't have much time to investigate, but it cannot find migrations where they really are. I rolled back to 0.5.x and it helped.

P.S. We probably should move it to `peerDependencies` and do not use/keep local copy.