Fix issue with sslverify if set to false.

There should be a difference if @config.sslverify is set to false
instead of nil. In the latter case we should print "false" as we are
explicitly telling this repository to setup verification.

Also changes the [setting to True/False as per documentation][1].

[1]: http://linux.die.net/man/5/yum.conf
Merge pull request #88 from johnbellone/master

Fix issue with sslverify if set to false.