import go-git-fixtures using gopkg.in fails in tests

[ajnavarro]

We need to rewrite this line to make it works: https://github.com/src-d/go-git-fixtures/blob/master/fixtures.go#L281

[smola]

Instead of rewriting it, let's use gopkg.in first, then fallback to github.com/src-d. This way we maintain higher backwards compatibility.