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

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)...
Merge pull request #83 from mfojtik/fix-rpm-constraints

Merged by openshift-bot