Provide es5 build

[abhishiv]

ATM the package throws up webpack since webpack.optimize.UglifyJsPlugin can't process node_modules/sequential-task-queue/dist/lib/sequential-task-queue.js because it has es6 class.

I guess it would also break in older browsers which don't support es6 semantics.

Users of this module can fix this error by including node_modules/sequential-task-queue in their webpack config so it gets compiled, but maybe it's better that the module ships an es5 build?