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. (Introduced by #20.)

Fix the configuration of `requireToken` for the swagger remotable,
so that it works when loopback-explorer is not installed too. (Introduced by #25.)

/to: @ritch 
/cc: @raymondfeng