fix NAMESPACE to work with current bioc-devel pkgs

As of bioc-2.14, many of the alignment functions from GenomicRanges and/or Rsamtools have been split out into the GenomicAlignments package.  One of them is 'readGAlignmentsFromBam'.  This patch fixes the import rules and thereby allows the 'efficient' version of derfinder to install under the current bioc-devel branch.