Updated dependencies

[twsl]

Might improve .NET Standard 2.0 support

Checklist

• I have included examples or tests
• I have updated the change log
• I am listed in the CONTRIBUTORS file
• I have cleaned up the commit history (use rebase and squash)

Changes proposed in this pull request:

• Updated to latest RC version of LiGit2Sharp
• Updated all other nuget dependencies
• Updated Framework Versions (required by LibGit2Sharp) including the update of .NET Core version to LTS
• Attempt to simplify travi CI script
• Used VS .gitignore

[asbjornu]

As GitVersion stopped using GitTools.Core in GitTools/GitVersion#1581, this PR is unfortunately not relevant anymore and the whole repository will be archived. Please try the latest build of GitVersion and see if it solves whatever problem you had when you submitted this PR. If the problem persists, please submit a new issue or PR against the GitVersion repository.