Revert Backbone typings to 1.4.1

The 1.4.9 typings cause compilation errors, which are not related to the merge of 7.x into master.

Author: jasongrout

jupyterlab_widgets/package.json

@@ -70,7 +70,7 @@
     "@lumino/properties": "^1.1.0",
     "@lumino/signaling": "^1.2.0",
     "@lumino/widgets": "^1.3.0",
-    "@types/backbone": "^1.4.1",
+    "@types/backbone": "1.4.1",
     "jquery": "^3.1.1",
     "semver": "^6.1.1"
   },

packages/base/package.json

@@ -36,7 +36,7 @@
     "@lumino/coreutils": "^1.2.0",
     "@lumino/messaging": "^1.2.1",
     "@lumino/widgets": "^1.3.0",
-    "@types/backbone": "^1.4.1",
+    "@types/backbone": "1.4.1",
     "@types/lodash": "^4.14.134",
     "backbone": "1.2.3",
     "jquery": "^3.1.1",