Make Rakefile work with RSpec 3.1

RakeTask in RSpec 3.1 does no longer allow to specify an array of paths
as the pattern. Since the purpose of the problematic rake task is only
to exclude the integration test we use the new "exclude_pattern" method
to achieve the same result instead.
Merge pull request #187 from SUSE/review_140905_fix_rakefile_in_rspec_3_1

Make Rakefile work with RSpec 3.1