Update minimum juggler version to v2.7.0

This ensures that `dataSource.ping()` method handles connector errors
correctly. See the following GH issues for more details:

https://github.com/strongloop/strong-studio/issues/135
https://github.com/strongloop/loopback-datasource-juggler/pull/260

Close https://github.com/strongloop/strong-studio/issues/135

/to @ritch please review