Cannot run test execution using watir/taza/rspec

I'm using Unix system and trying to run Watir/Taza/RSpec test execution.

I just perform commands for creating taza structure, site, homepage and flow:

>     taza create structure
>     taza site google
>     taza page home google
>     taza flow search google

Then I run test using next command:

rake spec:isolation:google

but I get an error:

rake aborted! LoadError: cannot load such file -- taza/tasks /Users/ismotrov/works/test/Rakefile:3:in require' /Users/ismotrov/works/test/Rakefile:3:in' /Library/Ruby/Gems/2.0.0/gems/rake-10.4.2/lib/rake/rake_module.rb:28:in load' /Library/Ruby/Gems/2.0.0/gems/rake-10.4.2/lib/rake/rake_module.rb:28:in load_rakefile' /Library/Ruby/Gems/2.0.0/gems/rake-10.4.2/lib/rake/application.rb:689:in raw_load_rakefile' /Library/Ruby/Gems/2.0.0/gems/rake-10.4.2/lib/rake/application.rb:94:in block in load_rakefile' /Library/Ruby/Gems/2.0.0/gems/rake-10.4.2/lib/rake/application.rb:176:in standard_exception_handling' /Library/Ruby/Gems/2.0.0/gems/rake-10.4.2/lib/rake/application.rb:93:in load_rakefile' /Library/Ruby/Gems/2.0.0/gems/rake-10.4.2/lib/rake/application.rb:77:in block in run' /Library/Ruby/Gems/2.0.0/gems/rake-10.4.2/lib/rake/application.rb:176:in standard_exception_handling' /Library/Ruby/Gems/2.0.0/gems/rake-10.4.2/lib/rake/application.rb:75:in run' /Library/Ruby/Gems/2.0.0/gems/rake-10.4.2/bin/rake:33:in' /usr/bin/rake:23:in load' /usr/bin/rake:23:in'

Who knows what to do? And another moment I would like to dicuss - in file config/config.yml I have the following:

---
browser: firefox
driver: watir_webdriver

But on my Mac I don't have Firefox installed. How can I use this with, for example, Chrome and can It cause my problem?

Thanks in advance!