Bump submodules to *-RC2 or master

This bumps all submodules to *-RC2 or later. Chisel and FIRRTL are
bumped to master.

This is now safe as submodules are only used to grab a README to
populate sections of the website. Submodules are no longer used as sbt
project dependencies.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>