fix(main): update main to transpiled version (#42)

If someone wants to require('cross-env') let it be es5

package.json

@@ -2,7 +2,7 @@
   "name": "cross-env",
   "version": "0.0.0-semanticallly-released.0",
   "description": "Run commands that set environment variables across platforms",
-  "main": "src/index.js",
+  "main": "dist/index.js",
   "bin": {
     "cross-env": "bin/cross-env.js"
   },