Changed default view engine to pug

[ghost]

npm says that jade package includes some security vulnerabilities which is one of them is critical.
so, I changed the default engine immediately.
I didn't tested the app, since I don't know how to do it.

[ghost]

Changed default view engine from jade to pug.
Also adjusted tests.

[sam-github]

I'd like to see this change as well, having security vulns in the default use case isn't something that should be enouraged for express users (or any project).

@replacepreg this needs additional changes:

1. package.engines.node has to change to > 0.12
2. package.version has to change to 5.0.0 (because dropping support for a node version is semver-major)
3. .travis.yml needs 0.10 and 0.12 removed

Maintainers - opinions?

https://www.npmjs.com/package/jade has been thorougly deprecated, its not a good default.