Update Git to 2.9

Ubuntu 16.x comes with Git 2.7.x. Azure DevOps requires at least Git 2.9 for checking out sources.

It should either be documented that the image running on Ubuntu 16.x just supports Azure DevOps until a specific version, or the image should install Git from another repository. 

E.g. something like:

```
add-apt-repository ppa:git-core/ppa
apt-get update
apt-get install git
```