Using let limits browser and node support

[atkinchris]

This commit (2296030) replaces var with let for ES2015 style. However, the let syntax is not widely supported, and breaks with some toolsets, such as uglify.

This could be resolved by:

• Reverting to supported syntax of var, or
• Transpile the npm published version to ES5.

[sindresorhus]

This module mainly targets Node.js. If you want to use it in the browser it's up to you to resolve it, like transpiling it with Babel.

[futpib]

What about publishing a transpiled file and adding a browser field to package.json for easy usage with browserify/webpack? Would such PR be welcome or not?