(PUP-1735) Reimplement deprecated Puppet::Node::Environment.current

Puppet::Node::Environment.current is used in some external code, so
reimplement it, linking to the context lookup for the current environment
and issue a deprecation warning.
(pup-1505) Correct require

The previous patch for pup-1505 changes to use a facter
top-level method (rather than one buried inside facter) but
neglected to update the require to match. This patch updates
the require.
Merge pull request #2384 from kylog/issue/pup-1505/fix-require

(pup-1505) Correct require
Merge pull request #2382 from domcleal/tickets/master/pup-1735-current-env

(PUP-1735) Reimplement deprecated Puppet::Node::Environment.current
Merge branch 'master' of https://github.com/puppetlabs/puppet