Fix a bug introduced in #25

Setting a googleanalytics_fields key to `True` as in #25 causes the
resulting Javascript to also include the literal `True`, which is not
valid. This works around that by setting the key to `'true'` which
when evaluated in an if statement is truthy, serving the same purpose

ckanext/googleanalytics/plugin.py

@@ -72,7 +72,7 @@ def configure(self, config):
             'googleanalytics.linked_domains', ''
         ).split(',')]
         if self.googleanalytics_linked_domains:
-            self.googleanalytics_fields['allowLinker'] = True
+            self.googleanalytics_fields['allowLinker'] = 'true'
 
         self.googleanalytics_javascript_url = h.url_for_static(
                 '/scripts/ckanext-googleanalytics.js')