Use yarn in ember-try.

Author: chriskrycho

config/ember-try.js

@@ -1,21 +1,22 @@
 /* eslint-env node */
 module.exports = {
+  useYarn: true,
   scenarios: [
     {
       name: 'ember-lts-2.12',
       npm: {
         devDependencies: {
-          'ember-source': '~2.12.0'
-        }
-      }
+          'ember-source': '~2.12.0',
+        },
+      },
     },
     {
       name: 'ember-lts-2.16',
       npm: {
         devDependencies: {
-          'ember-source': '~2.16.0'
-        }
-      }
+          'ember-source': '~2.16.0',
+        },
+      },
     },
     {
       name: 'ember-release',