Python 3.5 is out

[hugovk]

Also:

• Sudo is not used, so use container-based infra for faster builds
• Allow builds on all branches, so feature branches are also tested.
• Add CI build badge.

[rth]

@hugovk If this PR is superseded by PR #20, maybe it could be closed?

[hugovk]

@rth Two PRs give the maintainers the option of merging just the subset if they prefer. Although I think #20 would be good too :)