Delay the pkgwat exception via Component.verify.

Tested this in combination with #101 and confirmed the LoadError is no longer raised and is instead delayed until you try to use the corresponding classes, see below:

```
$ bundle console
Resolving dependencies...
Failed to require gem2rpm.  Added runtime exception in Polisher::RPM::Requirement
Failed to require gem2rpm.  Added runtime exception in Polisher::RPM::Spec
Failed to require pkgwat.  Added runtime exception in Polisher::Bodhi
Failed to require pkgwat.  Added runtime exception in Polisher::RHN
Failed to require pkgwat.  Added runtime exception in Polisher::Fedora
irb(main):001:0> Polisher::Bodhi.new
RuntimeError: polisher is missing a dependency - cannot instantiate
```

Note, this branch is independent of #101 and can be merged independently.