Fix jump-to-source (Fixes #1)

Once boot [2.0.0-rc4](https://github.com/boot-clj/boot/milestones/2.0.0-rc4) is released, vim-boot should be back to a good place. For the mean time, you can `make install` [my fork of boot](https://github.com/rkneufeld/boot) to test.

Fixes #1