Make sure latest libressl gets installed by Homebrew

We weren't telling people to run `brew update` before doing
`brew install libressl`. This could result in their using an older
version of libressl than we build against if they hadn't updated
their index in a while.

Author: SeanTAllen

README.md

@@ -13,6 +13,7 @@ This project is currently beta software.
 #### macOS
 
 ```bash
+brew update
 brew install libressl
 ```