fallback/default_branch takes priority over branch/ref/tag

[igt-marcin-wasilewski]

Is that intended that Puppetfile containing

mod 'example_module',
  :git => 'git@somehost.com/foo/example-module.git',
  :branch => 'foo',
  :fallback => 'b | a| r|',

would not checkout branch foo but try one of fallbacks first?

That's opposite from original r10k behavior.

https://github.com/xorpaul/g10k/blob/f91099adb1071e566a720478eb929d32d243880d/puppetfile.go#L396-L418

Potential fix: #248