Lock the MathQuill version installed by npm.

With breaking changes coming to MathQuill, PG-2.17 needs to be locked
into using the current version of MathQuill.  This locks PG-2.17 to
the WeBWorK-2.17 tag in our MathQuill repository.

Author: drgrice1

htdocs/package-lock.json

@@ -14,7 +14,7 @@
         "jsxgraph": "^1.4.2",
         "jszip": "^3.10.1",
         "jszip-utils": "^0.1.0",
-        "mathquill": "github:openwebwork/mathquill",
+        "mathquill": "github:openwebwork/mathquill#WeBWorK-2.17",
         "x3dom": "^1.8.1"
     },
     "devDependencies": {