Enable continuous deployment to nuget.org

[Florian Hockmann]

We can enable deployment simply via Travis, for example every time we tag a commit (with a release number). This requires the following steps:

• Create a JanusGraph account on nuget.org.
• Add necessary credentials for nuget.org (probably an API key) as an encrypted variable to Travis.
• Let Travis push the NuGet package on master if the commit has a tag.