try to fix travis builds

- removed 'composer' rule from 'prepare' rule dependencies (ie. it can
  be run independently from the rest of the build)
- change .travis.yml accordingly to run 'composer' from before_install
  step.
- added tools/get_composer.sh in order to get composer from upstream if
  it's not already installed and update it to the latest version if
  already installed.
- updated phpunit/README.asc according to those changes.
removed our copy of composer

We do not need this anymore since there is now a script that download it
directly from upstream.