Relying on the version of "roslib" package instead of the version
of "ros" stack.

 * The roslib API break required to the ROS message generation script
   to generate different ROS messages according to the different ROS
   distributions.

 * The ROS distributions were found by checking the version of the
   "ros" stack. In ROS Groovy "ros" stack became a meta-package and
   lost its version number.

 * Starting from now the ROS message generation script will relly
   straight on "roslib" version. It should be identical with the
   "ros" version because it was contained by "ros" stack.