Update references to MathJax to version 2.7.5

Author: dpvc

README.md

@@ -62,7 +62,7 @@ The `config` method is used to set _global_ configuration options. Its default o
   displayErrors:   true,     // determines whether error messages are shown on the console
   undefinedCharError: false, // determines whether "unknown characters" (i.e., no glyph in the configured fonts) are saved in the error array
   extensions: '',            // a convenience option to add MathJax extensions
-  fontURL: 'https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.2/fonts/HTML-CSS', // for webfont urls in the CSS for HTML output
+  fontURL: 'https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/fonts/HTML-CSS', // for webfont urls in the CSS for HTML output
   paths: {},                  // configures custom path variables (e.g., for third party extensions, cf. test/config-third-party-extensions.js)
   MathJax: { }               // standard MathJax configuration options, see https://docs.mathjax.org for more detail.
 }