Use 'rpmspec' command to get list of requires in tito.rb

**NOTE**: This needs some testing... I was looking for some tests but seems like there are none ;-) So **please** test this before push... 

This will replace parsing of the RPM spec file in Ruby by
parsing it using the 'rpmspec' command.

The rpmspec command provides better results and it also counts with
the RPM contraints (%if, etc)...