Remove duplicate precommit-hook dependency. 

This commit removes the duplicate `precommit-hook` dependencies and saves it as a dev dependency only. Having this as a production dependency causes the git hook to be written to the project which requires this module.

It looks like #4 fixed the dependency location issue, but it accidentally got re-introduced and then duplicated with [this commit](https://github.com/AmpersandJS/ampersand-router/commit/15833a52fa3e2a0300e74211d657bf7b14c1fb9d
).