Don't check for newer available versions in dashboard for PE

Prior to this commit, the PuppetDB dashboard was checking for
newer versions of PuppetDB even if we are running inside of
a PE environment; this commit simply prevents the code
from checking for newer versions when we're in PE, and always
tells the dashboard that there are no newer versions available.
Add missing keys to latest version response for PE

Previous commit was returning a hash that only contained
`"newer" => false` for PE version check.  This commit
adds in the `version` and `link` keys.
Merge pull request #517 from cprice-puppet/bug/PE-286/dont-show-updates-in-dashboard-for-pe

Don't check for newer available versions in dashboard for PE