Add a check for yarn versions.

For non-JS people in a Ruby project it can be very confusing if JS isn't their normal ecosystem. This commit provides a message to positively know what is wrong with relation to yarn versions.

package.json

@@ -71,6 +71,7 @@
   },
   "engines": {
     "node": ">= 6.9.1",
-    "npm": ">= 3.10.3"
+    "npm": ">= 3.10.3",
+    "yarn": ">= 1.0.0"
   }
 }