Updated to libgit2sharp 0.18.1.0

Hi,

Thanks for a great tool :+1: 

Due to that I'm running your server on a webserver at unoeuro I had a problem creating a new repository.

I tracked it down to something wrong with libgit2 and created a thread here https://github.com/libgit2/libgit2/issues/2486

But in the meantime I updated all the libs to see if that helped, but unfortunately it didn't - but I might as well share the update.

I tried to run the test, but two of them are failing 

```C#
[TestMethod]
        public void HandlesCshtmlExtension()
        {
            Assert.AreEqual(PathEncoder.Encode("index.cshtml"), "index.cshtml.view");
            Assert.AreEqual(PathEncoder.Decode("index.cshtml.view"), "index.cshtml");
        }
```

and Mysysintegrationtest - due to a lot of local paths?

Hope you can use it,

Lars Glud