Fix loopback-explorer and swagger setup.

Correctly detect when loopback-explorer was not installed. Before,
the condition used to detect presence of loopback-explorer was always
evaluated as true.

Fix the configuration of `requireToken` for the swagger remotable,
so that it works when loopback-explorer is not installed too.
Merge pull request #26 from strongloop/feature/fix-swagger-setup

Fix loopback-explorer and swagger setup.