Add loopback-datasource-juggler as devDependency

Because loopback lists loopback-datasource-juggler as a peerDependency but
still require()s it directly, it must be present for tests to run.

Under normal usage, the loopback dependency would pull in
loopback-datasource-juggler. This is mainly for dev/CI.
Merge pull request #28 from strongloop/feature/fix-missing-dependency

Loopback expects loopback-datasource-juggler, but no longer requires it