Remove phantomjs from default installation instructions

Author: mdmintz

help_docs/before_installation.md

@@ -29,7 +29,7 @@ Mac:
 * On a Mac, you can install drivers more easily by using ``brew`` (aka ``homebrew``), but you have to install that first. [Brew installation instructions are here](https://github.com/seleniumbase/SeleniumBase/blob/master/help_docs/requirements_installation.md).
 
 ```bash
-brew install chromedriver phantomjs
+brew install chromedriver
 ```
 
 (NOTE: If your existing version of chromedriver is less than 2.32, **upgrading is recommended!**)