[WIP] Automatic dependency rebuilding: Update paths in travis.sh

Update paths in `travis.sh` to work with automatic dependency builds on Travis (see neovim/bot-ci#8). The last commit is a quick fix for the problems in #1031 so that builds won't fail because of that.

Would need to be merged after neovim/bot-ci#8 is merged and a `neovim/bot-ci` build has been successful. Until then, this PR will not build on Travis, because the updated paths are not yet in `neovim/deps`.

Related: #1033.
@fwalch This won't be needed if we update busted, right?