Update min version of hackney to 1.15.2 (#575)
Update min version of hackney to 1.15.2 (#575)

There's an issue with older versions of hackney that results in failed
HTTPS requests with this error:

```
{:error, {:option, :server_only, :honor_cipher_order}}
```

See https://github.com/thoughtbot/bamboo/issues/512.
thoughtbot/bamboo