Change loopback peer dep to "1.x || 2.x"

Support the upcoming 2.x version of LoopBack.

The devDependency is intentionally kept at 1.x to ensure the module
supports LoopBack 1.x too.
Merge pull request #3 from strongloop/feature/support-loopback-2.x

Change loopback peer dep to "1.x || 2.x"