Fix Uncaught Type Error

Fixes #167

Since version 0.4.9, we require `fs` but it should fail if we are in
a browser. However, `browser-source-map-support.js` is bundled with
browserify which converts unsupported builtins to empty object. See
https://github.com/substack/node-browserify/blob/8f5ebbf505/lib/builtins.js#L12
This PR checks if required `fs` contains `readFileSync` which is part
of `fs` API.

source-map-support.js

@@ -4,6 +4,10 @@ var path = require('path');
 var fs;
 try {
   fs = require('fs');
+  if (!fs.readFileSync || !fs.readFileSync) {
+    // fs doesn't have all methods we need
+    fs = null;
+  }
 } catch (err) {
   /* nop */
 }