Fixes Waterline issue 346 by syncing (copy+paste) sails-disk
https://github.com/balderdashy/waterline/issues/346

Looks like the sails-disk got updated for some changes whereas sails-memory didn't.

I more-or-less copy+pasted the apparently-relevant changes from sails-disk to sails-memory
et voila, it passes the tests and no longer causes problems in my own project.
Adds Travis config file. Let's see those builds go green.
I hope.
Merge pull request #9 from otupman/waterline-346-push-undefined

Fixes issue #8 by syncing (copy+paste) sails-disk