Installation requirements for MacOS

Proposing to add installation of "gettext" package via brew for installing on mac os, since "envsubst" seems not to be included in mac's bash by default. envsubst is required by the installation scripts.

```
brew install gettext
brew link --force gettext 
```

Taken from: https://stackoverflow.com/questions/23620827/envsubst-command-not-found-on-mac-os-x-10-8

