Use a more flexible rubygem requirement syntax (bnc#895069)

https://en.opensuse.org/openSUSE:Packaging_Ruby#Format_of_automatically_generated_Provides:_headers
Merge pull request #162 from yast/rubygem-requires

Use a more flexible rubygem requirement syntax (bnc#895069)
sort displayed addons (bnc#888567)
use the custom URL also in upgrade (bnc#894592)
added some logging, continue after registring the system
avoid SLP discovery if the default SCC server was already used for registration
added exceptions_spec.rb test
changes