Revert "(FACT-658) facter doesn't parse gnu uptime output"

This reverts commit b8ea20db6826c906f9c527a7dbf9aa04fbfae960.

After some discussion with Josh Cooper, we agreed that relying
on the exact path to uptime for all platforms is the wrong approach,
so we're reverting.

Two possibilities are:
1) Set the exact path to uptime for Solaris only
2) Make the uptime parsing resilient to gnu uptime output