Configure the build matrix

[jubianchi]

• Remove hhvm and hhvm-nightly from the matrix and allowed failures
• Add two environments for composer : --prefer-stable and --prefer-lowest
• Merge before_script and script
• Move atoum/atoum to the require section in composer.json
• Fix the version constraint on atoum/atoum: it should be at least ^2.0
• The PHP nightly should be an allowed failure

--prefer-lowest may not pass on php >= 7.0: in this case, disabling --prefer-lowest on php >= 7.0 is OK.

See here fro an example

[michaelspiss]

I somehow messed up the pull request as it did not assign me to this issue...
Sorry for the inconvenience, I'm a first-time contributor