Remove repos from the main `bundle` call. Fix bugs.

* `bundle` now simply uses the default repositories, meaning the user
 must set their repos option prior to calling `bundle`. This greatly
 simplifies the logic in `bundle`, since we no longer have to deal with
 the temporary repository setting and the `tryCatch` surrounding it.
 This also makes it far easier to debug.
* Refactor `install_version` to make it's functionality dead-simple. Remove
nearly all of the original implementation of install_version
* Export `find_available_versions`. Have it create a data.frame with the
source and URL of the package to install, instead of just the version.
* Set pkgType to 'source' for all testing.