Mention `renew` deprecation in vault section in changelog

When porting a configuration file from an old version of envconsul (`0.6.2`) to `0.7.3`, I got this error

```
mapstructure decode failed: 1 error(s) decoding:
* 'vault' has invalid keys: renew
```

I guess than when you are working with vault intimately every day, it is very obvious that this is simply `renew_token`, but I was not quite sure.

When looking at the [CHANGELOG.md](https://github.com/hashicorp/envconsul/blob/master/CHANGELOG.md) I did not find any mention of `renew`.

I digged through the source code of `0.6.2` to find this snippet

https://github.com/hashicorp/envconsul/blob/7ece7357db8003233806d4169a4fa5d3f36de0a8/vendor/github.com/hashicorp/consul-template/config/config.go#L234-L241

Would be super nice to mention the deprecation of `renew` in the changelog :) _I'm not sure when this happened, therefore this is not a PR that fixes it._

### Envconsul version
`0.7.3`

### Configuration
```hcl
vault {
renew = true
}
```

### Command
```shell
envconsul -config
```