Update index.spec.tsx

Reverting from 3.1.0 to 2.7.9

Author: stormpython

Diff for: __tests__/index.spec.tsx

@@ -6,7 +6,7 @@ jest.mock("load-script");
 import loadScript from "load-script";
 import { Node, Provider, Text } from "../src";
 
-const mathJaxUrl = "https://cdnjs.cloudflare.com/ajax/libs/mathjax/3.1.0/es5/tex-chtml-full.min.js";
+const mathJaxUrl = "https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.9/MathJax.js?config=TeX-MML-AM_CHTML";
 
 describe("MathJax", () => {
   test("Node can be renderered without provider", () => {