Fix license validation

[btzr-io]

I don't really know how well this will perform neither have any experience in go. But this should give you an idea of how to fix the current issue.

Playground test:
https://play.golang.org/p/5HvK33uGmyi

Please take a look at other alternative methods:
https://stackoverflow.com/questions/28718682/how-to-get-a-substring-from-a-string-of-runes-in-golang

Issue: #164

[codeclimate]

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