Switch MathJax script to CDNJS

Close #789.

Author: uranusjr

MacDown/Code/Document/MPRenderer.m

@@ -21,7 +21,7 @@
 
 
 static NSString * const kMPMathJaxCDN =
-    @"https://cdn.mathjax.org/mathjax/latest/MathJax.js"
+    @"https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js"
     @"?config=TeX-AMS-MML_HTMLorMML";
 static NSString * const kMPPrismScriptDirectory = @"Prism/components";
 static NSString * const kMPPrismThemeDirectory = @"Prism/themes";

MacDown/MacDown-Info.plist

@@ -92,7 +92,7 @@
 		<true/>
 		<key>NSExceptionDomains</key>
 		<dict>
-			<key>mathjax.org</key>
+			<key>cdnjs.cloudflare.com</key>
 			<dict>
 				<key>NSExceptionRequiresForwardSecrecy</key>
 				<false/>