[HOTFIX] Pin @lumino/coreutils to 1.5.0 to avoid build issues (#686)

More details at: jupyterlab/lumino#83

Author: Martha Cryan

package.json

@@ -52,6 +52,7 @@
     "react-dom": "~16.8.5",
     "@types/react-dom": "~16.8.5",
     "react-intl": "^2.3.0",
-    "@types/react-intl": "^2.3.0"
+    "@types/react-intl": "^2.3.0",
+    "@lumino/coreutils": "1.5.0"
   }
 }