swagger: Deprecate `opts.swaggerVersion`

Users of loopback-explorer should not override the swagger version,
as it's the explorer who decides what version of the Swagger Spec
it implements.

lib/swagger.js

@@ -23,6 +23,9 @@ var cors = require('cors');
  * @param {Object} opts Options.
  */
 function Swagger(loopbackApplication, swaggerApp, opts) {
+  if (opts && opts.swaggerVersion)
+    console.warn('loopback-explorer\'s options.swaggerVersion is deprecated.');
+
   opts = _defaults(opts || {}, {
     swaggerVersion: '1.2',
     basePath: loopbackApplication.get('restApiRoot') || '/api',