Use bash for install and switch

First off thanks for the module!

This PR fixes a problem when installing on a system that is using sh as the base shell and enforces the usage of bash. 
It also fixes some annoyances when installation fails in `perlbrew_init_file` and makes it executable.

I'm using this branch on ubuntu 14.04 and it work like a charm so if you want I can add a PR so add ubuntu support.