JS: update Readmore, change call syntax

Update Readmore to v3.0.0-beta-1 (latest release). v3 is no longer a jQuery plugin, so the GBL item module needs to call the feature via: new Readmore(...). Add simple js spec to test Readmore is defined.

Fixes #800

app/assets/javascripts/geoblacklight/modules/item.js

@@ -9,7 +9,5 @@ Blacklight.onLoad(function() {
     viewer = new window['GeoBlacklight']['Viewer'][viewerName](element);
   });
 
-  $('.truncate-abstract').readmore({
-    maxHeight: 60
-  });
+  new Readmore( '.truncate-abstract', { maxHeight: 60 } );
 });

lib/generators/geoblacklight/templates/geoblacklight.js

@@ -7,4 +7,4 @@
 //= require geoblacklight/downloaders
 //= require leaflet-iiif
 //= require esri-leaflet
-//= require readmore.min
+//= require readmore

spec/javascripts/geoblacklight_spec.js

@@ -13,5 +13,9 @@ describe('GeoBlacklight', function() {
     it('MetadataDownloadButton is defined', function() {
       expect(GeoBlacklight.MetadataDownloadButton).toBeDefined();
     });
+
+    it ('Readmore is defined', function() {
+      expect(Readmore).toBeDefined();
+    });
   });
 });