Upgrade to nginx 1.8

[philpep]

This patch work on a new server but not a already provisioned because nginx is already installed and not upgraded.

Travis build: https://travis-ci.org/philpep/test-driven-infrastructure-example/builds/76951116
Jenkins build: https://jenkins.philpep.org/job/test-driven-infrastructure-example-pr/14/

    def test_package(Package):
        nginx = Package("nginx")
        assert nginx.is_installed
>       assert nginx.version.startswith("1.8")
E       assert <built-in method startswith of unicode object at 0x7fd13eee2fc0>('1.8')
E        +  where <built-in method startswith of unicode object at 0x7fd13eee2fc0> = '1.4.6-1ubuntu3.3'.startswith
E        +    where '1.4.6-1ubuntu3.3' = <package nginx>.version

test_nginx.py:7: AssertionError