Update octokit icon for nuget package

Fixes #202 

Here's how it looks at the three sizes that NuGet displays it (first 2 on nuget.org and the last one in the VS extension)

128x128
![octokit-128x128](https://f.cloud.github.com/assets/19977/1510987/64af2b26-4a9d-11e3-89fc-96a185171c75.png)
48x48
<img src="https://f.cloud.github.com/assets/19977/1510987/64af2b26-4a9d-11e3-89fc-96a185171c75.png" width="48" />
32x32
<img src="https://f.cloud.github.com/assets/19977/1510987/64af2b26-4a9d-11e3-89fc-96a185171c75.png" width="32" />