resolve #232: open browser linux compatibility

@kevgo 

Able to test by overriding the `which` command to fake what tools are installed on the system.
I don't see a need to specify that the open tool was used to open the browser. An end-user only cares that he has to have the "open" tool in order to see browser windows. He doesn't care whether or how exactly Git Town uses that command.

Given that, I think the best strategy here is to leave the tool name out of the `Then` line, store the tool name in a variable in the `Given` line, and use that for comparison.