Require candlepin-common

This patch prepares candlepin to be built against the new candlepin-common rpm.

TEST
-------
* Build a SRPM
```tito build --test --srpm```
* Do an el7 scratch build to verify it builds for you
```brew build --nowait --scratch candlepin-1-rhel-7-candidate /path/to/candlepin.src.rpm```
* Do an el6 scratch build to verify it builds for you
```brew build --nowait --scratch candlepin-1-rhel-6-candidate /path/to/candlepin.src.rpm```

What doesn't work yet is building candlepin against fedora for the community builds nor building in katello-koji as candlepin-common is not there yet.