[RFC] Update APT package sources in travis.sh.

Apparently, there has been an updated package in the APT repos, which causes `api/python` builds to [fail](https://travis-ci.org/neovim/neovim/jobs/31790494) with

> Err http://us.archive.ubuntu.com/ubuntu/ precise-updates/main libc6-dev amd64 2.15-0ubuntu10.5
> 404 Not Found [IP: 2001:67c:1562::13 80]

Current version in the repos is 2.15-0ubuntu10.**6**; executing `apt-get update` before installing packages should resolve this. Also fixed indentation.