mpich2: Update for 3.1rc2 release

Add the information for the 3.1rc2 release as a devel tag. This is the first
time this formula has an unstable version so some new logic needed to be added
to cover these cases.

This release changes the default behavior from disabling shared libraries to
enabling them so there needs to be another flag to cover this case. For now,
both options will be available to the user (MPICH will ignore the flag if it
is redundant). When 3.1 is fully released, the --enable-shared flag will be
removed entirely as it will always be redundant.