Bump up minimum required version of juggler

Require at least 2.6.1 to ensure the method `DataSource.prototype.ping`
is available.
testConnection: use ping, run in workspace app

Rework `DataSourceDefinition.prototype.testConnection` to invoke `ping`
method in the workspace app.

Improve error handling in `invokeMethodInWorkspace` to catch common
errors and translate them to a useful error in the master process.

Mark `DataSourceDefinition.testConnection` as deprecated, implement
it using the original `testConnection` code running in the master
process.
Merge pull request #124 from strongloop/feature/test-connection-using-ping

Test connection using ping