Change default branch name from master to main

If the "Git branch, tag, or commit" field is not filled we will now
default to main instead of master

Author: sgibson91

binderhub/static/js/index.js

@@ -121,7 +121,7 @@ function getBuildFormValues() {
     repo = encodeURIComponent(repo);
   }
 
-  var ref = $('#ref').val().trim() || 'master';
+  var ref = $('#ref').val().trim() || 'main';
   if (providerPrefix === 'zenodo' || providerPrefix === 'figshare' || providerPrefix === 'dataverse' ||
       providerPrefix === 'hydroshare') {
     ref = "";