Fix installing templates from internal repositories

`new.clj` was assuming an old map format for repositories.  This mean that repositories stored in `~/.lein/profiles.clj` in the recommended format (vectors of vectors) would make the `merge` throw, and line 28 would swallow the exception.

This uses `lein.core.project`'s already-defined default repositories and method of merging them.  AFAIK, this should handle any valid way of specify plugin repositories.

Using this, I can install templates from an internal repository specified in my `profiles.clj`.