Use asyncio.ensure_future instead of asyncio.async
Use asyncio.ensure_future instead of asyncio.async

`async` is a reserved word in python 3.7. Fortunately, asyncio has
renamed the function back around python 3.4 to `ensure_future`. The
`async` alias was deprecated back in 3.5. Switch to the new function
name.

Fixes #1

kepstin/regainer