Fix undefined rubygems version

Traceback (most recent call last):
  ...
    recipe.install()
  File "rubygemsrecipe/rubygems.py", line 347, in install
    gem_executable):
  File "rubygemsrecipe/rubygems.py", line 300, in get_dependency_list
    if self.version[0] < '3':
TypeError: 'NoneType' object has no attribute '__getitem__'

The test that was added in 60b10788 is adapted due to the call to subprocess.checkout .

See merge request !7