Merge pull request #1764 from AbdealiJK/ajk/codemirror

package.json: Update CodeMirror to 5.23+

package.json

@@ -10,7 +10,7 @@
     "@jupyterlab/services": "^0.38.0",
     "ansi_up": "^1.3.0",
     "backbone": "^1.2.0",
-    "codemirror": "^5.20.2",
+    "codemirror": "^5.23",
     "d3-dsv": "^1.0.0",
     "diff-match-patch": "^1.0.0",
     "es6-promise": "^3.2.1",