update travis to build all php versions.

[glensc]

This serves also as mini HOWTO, how to test with phpunit with php versions 5.3 to 7.3 in travis.

1. using travis build stages
2. hhvm marked as allow failure as it's not goal to support it
3. all builds but 5.3 run on trusty, required by build stages afaik
4. 5.3 runs on precise because it doesn't work on trusty
5. phpunit versions provided by travis image are inconsistent, \PHPUnit\Framework\TestCase class not provided consistently.
6. using composer require without version requirement will pick best phpunit version for the environment
7. while system phpunit will use actually code from vendor if present, it's --version arg reports system phpunit version, so use phpunit from vendor for clarity

also here's overview of phpunit versions installed on travis:

• 5.3: 4.8.18
• 5.4: 4.8.35
• 5.5: 4.8.27
• 5.6: 5.7.21
• 7.0: 6.3.0
• 7.1: 6.4.0
• 7.2: 6.4.1
• 7.3: 6.3.0

and after using from composer:

• 5.3: 4.8.36
• 5.4: 4.8.36
• 5.5: 4.8.36
• 5.6: 5.7.23
• 7.0: 6.4.2
• 7.1: 6.4.2
• 7.2: 6.4.2
• 7.3: 6.4.2