silence warnings about unescaped '-' in re char class

Eliminate warnings like this:

vagrant/plugins/hosts/linux/cap/nfs.rb:74: warning: character class has '-' without escape: /^# VAGRANT-BEGIN:( 1000)? ([\.\/A-Za-z0-9-_]+?)$/
Merge pull request #4713 from aspiers/silence-nfs-warnings

silence warnings about unescaped '-' in re char class