Enable optimized builds on Travis

[larsbergstrom]

By default, Cargo builds unoptimized. This changes the default on Travis to optimized builds, since we both find so many of our rust compiler bugs only in optimized builds and optimized is the flavor we would actually ship in the future.

r? @metajack

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/2579

This is an external review system which you may optionally use for the code review of your pull request.

In order to help critic track your changes, please do not make in-place history rewrites (e.g. via git rebase -i or git commit --amend) when updating this pull request.

[Manishearth]

Also, for some strange reason, no opt builds aren't much of an improvement speed-wise. If anything, they're slower!

[jdm]

The build-cef command does not accept the arguments: --release

[larsbergstrom]

I just saw that as well and was working on a patch to add the flag to the pile of python :-)

[SimonSapin]

It looks like we might want -O but not --cfg ndebug, so that debug_assert!() statements are run on Travis. https://critic.hoppipolla.co.uk/showcomment?chain=7610

We can configure this in Cargo.toml: http://crates.io/manifest.html#the-[profile.*]-sections

[larsbergstrom]

We no longer Travis the things.