Use `no_checksum` instead of a sha1 when getting `latest` version.

installation currently fails because of a SHA mismatch
```
> brew cask install mou
==> Downloading http://mouapp.com/download/Mou.zip
######################################################################## 100.0%
Error: SHA1 mismatch
Expected: e69bc2bbdadea6a2929032506085b6e9501a7fb9
Actual: 1276772fe649b2556257f6c0cf6e3ad5998fa35b
```

We've already dropped the explicit version. It seems to follow to drop the explicit SHA.