Bump loopback-datasource-juggler minimum version

The .ping support added in 2ffa87a and the .ping() support tests added
in 8d86533 require loopback-datasource-juggler@^2.6.0.
Expanded tests in test/default-scope.test.js added in afbfc75 require
loopback-datasource-juggler@^2.10.0.

/to @raymondfeng 
From https://github.com/strongloop/loopback-connector-mongodb/pull/59#issuecomment-61285378
Sorry, I accidentally did `--save` instead of `--save-dev`.