Exceeding 2 calls per second when using "theme upload"

I've been running into this for month now and haven't even noticed this, since it does not raise a critical error but only logs it and if you don't read every single line, you might very well miss it.

So I added the existing conditional sleep method to ShopifyTheme#send_asset and now it works [(pull request#244)](https://github.com/Shopify/shopify_theme/pull/224)