add last_run to the list of dynamic facts that are filtered out

The fact `last_run` comes from the `puppi` module which is fairly
common.  This patch also adds some simple tests for dynamic fact
filtering.  In the future, the list of fact filters is probably going to
need to be configurable via the API instead of hard coded into an ERB
template.
Merge pull request #88 from jhoblitt/filter_last_run_fact

add last_run to the list of dynamic facts that are filtered out