Allow native bcrypt for performance

See https://github.com/strongloop/loopback/issues/892
Merge tag 'v2.8.3'

2.8.3

 * Remove unused underscore dependency (Ryan Graham)
Merge pull request #894 from strongloop/feature/fix-issue-892

Allow native bcrypt for performance
v2.8.4
Merge branch 'release/2.8.4' into production