resolve #232: open browser linux compatibility

@kevgo 

Able to test by overriding the `which` command to fake what tools are installed on the system.
Please add a generic comment that describes what this file is for. Something like

```
# A test double for the system-wide `which` tool. 
# 
# Git Town is using `which` to discover system-wide command-line tools it depends on.
# This application returns hard-coded answers about whether these tools are installed or not.
# This allows simulating situations where no tools or only specific tools are installed, 
# and test those cases appropriately