Update composer to 1.0.0 once
Update composer to 1.0.0 once

Composer 1.0.0 beta 2 introduced the concept of channels in
`composer self-update`:
https://github.com/composer/composer/blob/master/CHANGELOG.md#100-beta2---2016-03-27
Before this feature, it always updated to the latest commit, which
could be buggy.

By stepping at 1.0.0, we ensure that `composer self-update` updates
to the stable release, avoiding a buggy commit.

JuliaCI/travis-build,kevmoo/travis-build,craigcitro/travis-build,kevmoo/travis-build,andyli/travis-build,craigcitro/travis-build,andyli/travis-build,JuliaCI/travis-build