Downgrade nouislider

With nouislider 15.5.0, there is a rendering issue with vertical sliders in Widgets List--the css rule in 15.5.0:

.noUi-vertical .noUi-origin {
  top: -100%;
}

seems to put the handle at the top of the slider when the slider is 0, and then the handle goes up beyond the slider for any higher values.

Author: jasongrout

packages/controls/package.json

@@ -43,7 +43,7 @@
     "d3-color": "^3.0.1",
     "d3-format": "^3.0.1",
     "jquery": "^3.1.1",
-    "nouislider": "15.5.0"
+    "nouislider": "15.4.0"
   },
   "devDependencies": {
     "@jupyterlab/services": "^6.0.0",