README: update Ginkgo install instructions

based on http://onsi.github.io/ginkgo/
put assertions inside an It()

> If you're using Ginkgo then you probably forgot to put your assertion in an It().
Merge pull request #7 from nathany/readme

README: update Ginkgo install instructions
Merge pull request #8 from nathany/fix_tests

Fix test failure: put assertions inside an It()
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