Update package.json to point 'main' at dist/katex.js (#791)
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
azotova/KaTeX,azotova/KaTeX,xymostech/KaTeX,kwangkim/KaTeX,azotova/KaTeX,xymostech/KaTeX,kwangkim/KaTeX,xymostech/KaTeX,azotova/KaTeX,Khan/KaTeX,kwangkim/KaTeX,azotova/KaTeX,kwangkim/KaTeX,xymostech/KaTeX,xymostech/KaTeX,Khan/KaTeX,kwangkim/KaTeX,Khan/KaTeX