Loosen tilt dependency to allow any 1.x version >= 1.3.3

This change allows slim to use tilt version 1.4.0 and any future 1.x version,
which ought to be compatible, as long as tilt adheres to Semantic Versioning.
Merge pull request #394 from sferik/loosen_tilt_dependency

Loosen tilt dependency to allow any 1.x version >= 1.3.3