Make require("6to5/register") work with browserify

Previously, you'd have to create a separate file for using 6to5
with both node and browserify, as the latter wasn't able to properly
handle loading 6to5's dependency tree and would crash on attempting
to do so.

This change instructs browserify to use "register-browser.js" in place
of "register.js". "register-browser.js" still loads the 6to5 polyfill,
but is otherwise ignored.

test/fixtures/browserify/register.js

@@ -0,0 +1,3 @@
+require('../../../register')({
+  ignoreRegex: false
+});