Update package.json to point 'main' at dist/katex.js (#791)

Summary:
Now that we're using ES6 import statements in our source code
it makes it difficult for people to consume the katex package
as a node module unless they're using node.js version 8.x.

This diff points the node entry to dist/katex.js which
is the compiled version of katex.js.

Test Plan:
- publish
- create a new project node project and add the new npm katex package as a deep
- require katex in a test file and run it using node.js version 6.x

package.json

@@ -2,7 +2,7 @@
   "name": "katex",
   "version": "0.8.2-pre",
   "description": "Fast math typesetting for the web.",
-  "main": "katex.js",
+  "main": "dist/katex.js",
   "repository": {
     "type": "git",
     "url": "git://github.com/Khan/KaTeX.git"