Instead of returning early, conditionally require actual runtime (./_runtime.js)

Fix this error with webpack:

ERROR in ./~/6to5/lib/6to5/transformation/transformers/generators/runtime.js
Module parse failed: /[...]/node_modules/6to5/lib/6to5/transformation/transformers/generators/runtime.js Line 12: Illegal return statement
You may need an appropriate loader to handle this file type.
|
| if (typeof global.regeneratorRuntime === "object") {
  |   return;
  |
}
|
 @ ./~/6to5/lib/6to5/polyfill.js 6:0-59
Remove global existence check
Deleted uneeded _runtime.js
Merge pull request #188 from brentburg/webpack-return

Webpack doesn't like return outside of a function in a module