change splitable to mergeable in MergeTableRegionsProcedure

  
private boolean isMergeable(final MasterProcedureEnv env, final RegionState rs)
  throws IOException {
    GetRegionInfoResponse response =
      Util.getRegionInfoResponse(env, rs.getServerName(), rs.getRegion());
    return response.hasSplittable() && response.getSplittable();
  }