Search license also in other branches

[Mte90]

As we can see

GH-License/ghlicense/repobase.py

Line 19 in 817ba6c

default_branch -- The branch to check (default "master").

we set the default branch.
But as today repository can have different as default, like for a fork and so on. We should find for all the repository behind services the "main" branch for a better check.