Debian: update apt database after adding the new source

Solves https://github.com/sensu/sensu-puppet/issues/201
Probably there are better ways of doing this.

I've actually had more issues w/ this vs the actual update not happening.  It seems to have updated, but without the linkage of the key & source, or, at least a :       before      => Package['sensu'] in the key stanza, it won't work.

There might be some stuff in the apt repos that can trigger apt-update on addition of new sources, not sure.