jQuery -> $ in src/metisMenu.js to fix import.

[BurkovBA]

First line of import.js imports jQuery as $. Accordingly, under $ name it should be visible to the rest of the module.

Thus, I replaced global jQuery in MetisMenu and Util with $ in constructor invocations and now metismenu will be successfully executed in node.js environment.

[BurkovBA]

Update: my previous built would've been broken in browser environment.

It required addition of babel-plugin-transform-es2015-modules-umd to fix renaming of global.jquery to global.jQuery in browser environment.Solution was suggested here: https://stackoverflow.com/questions/48818400/how-to-transpile-ecma6-import-of-jquery-to-a-umd-wrapper-with-babel/48824565#48824565

I didn't run an extensive testing so far though.

UPDATE: tested in browser with Webpack build - works fine now.