Install git-core for vagrant (smaller dependency) and add comment

Vagrantfile

@@ -8,7 +8,8 @@ VAGRANTFILE_API_VERSION = "2"
 $ubuntu_setup_script = <<SETUP_SCRIPT
 cd /vagrant
 sudo ./bootstrap/ubuntu.sh
-sudo apt-get -y --no-install-recommends install git
+sudo apt-get -y --no-install-recommends install git-core
+# the above is required by the 'git+https' lines of requirements.txt
 if [ ! -d "venv" ]; then
   virtualenv --no-site-packages -p python2 venv
   ./venv/bin/pip install -r requirements.txt -e .[dev,docs,testing]