Remove libxml2-dev package from development

Otherwise it fails when provisioning the VM:

```
Error: Duplicate declaration: Package[libxml2-dev] is already declared in file /vagrant/modules/performanceplatform/manifests/python_lxml_deps.pp:9; cannot redeclare at /vagrant/manifests/nodes.pp:49 on node development-1.localdomain

```
Merge pull request #471 from alphagov/fix-dev-provisioning

Remove libxml2-dev package from development