Fix development-1 Varnish configuration

Varnish was failing with:

```
vagrant@dev-development-1:~$ sudo service varnish start
 * Starting HTTP accelerator varnishd                                                                                                                                      [fail] 
Message from VCC-compiler:
Symbol not found: 'frontend_app' (expected type BACKEND):
('input' Line 46 Pos 25)
    set req.backend   = frontend_app;
------------------------############-

Running VCC-compiler failed, exit 1

VCL compilation failed
```
Merge pull request #437 from alphagov/fix-development-vcl

Fix development-1 Varnish configuration