Upgrade pointycastle dependency version to 3.1.1

[mattmaddux]

This Pull Request sets the pointycastle dependency version to a specific release to prevent unfixed errors when building for web.

Pull Request Checklist

• [X] My commits are in nice logical chunks with good commit messages
• [X] My changes are rebased on the latest trunk branch
• [X] Ran ´dartfmt -w lib test example´
• [X] All tests are passing
• [X] My changes are ready to be shipped to users

Issues affected

Fixes #27

[konstantinullrich]

I'll push a new Verison later today

[konstantinullrich]

Thank you for your contribution :)