Update dependencies

[kibertoad]

First and foremost this enables support for knex 0.19.0 (that should work out-of-the-box, since sources were moved from src to lib again), but also bumps several in-range dependencies in non-breaking way.

Closes #1988.

[kibertoad]

@ricardograca Do you think version 0.18 should be explicitly excluded from the supported range? (not sure if npm supports multiple intervals for peer dependencies, though)

[ricardograca]

Good question. I've played a bit with https://semver.npmjs.com/ and came up with this:

0.15.0 - 0.17.6 || ^0.19.0

I don't know if versions older than 0.15.0 still work, but if anyone still needs them they would have to use an older version of Bookshelf anyway since both Knex and Bookshelf dropped Node 4 support in the mean time.

[ricardograca]

Never mind. Excluding versions 0.18.x is only important for devDependencies but since we're upgrading to 0.19.0 explicitly there it's not an issue.

[kibertoad]

@ricardograca Are you saying that 0.18 will work fine with bookshelf in production? You only access internal dirs inside tests but not in production code?

[ricardograca]

Yes, that's it. Sorry about not realizing this sooner. We should stick to your previous commit in terms of peerDependencies. Do you mind excluding Node.js 6 jobs from Travis as well?

[kibertoad]

Done.

[ricardograca]

All green now! Thanks a lot 😃