Add additional paths for Ubuntu 14.04/Debian testing and MacPorts

This PR adds two default search paths to `FindPETSc.cmake`:

1. An additional directory for 3.4.2, since Debian testing and Ubuntu 14.04 now use this.
2. The path to the distribution provided by MacPorts.

I am hoping item 1 should be fairly unobtrusive. In terms of item 2, we are adding PETSc as an external dependency in another project in which OS X is a relatively common development environment, and this saves some effort in configuration. I thought I would add this into the PR in case you want to include it.