Rebuild Rmpi package for MPICH2

• Default debian package r-cran-rmpi is depended on lam4 not mpich/mpich2

  Source: rmpi
  Section: math
  Priority: optional
  Maintainer: Dirk Eddelbuettel <edd@debian.org>
  Build-Depends: debhelper (>> 4.1.0), cdbs, r-base-dev (>= 2.4.0), lam-runtime, lam4-dev (>= 7.1.1-3.2)
  Standards-Version: 3.7.2