Drupal 8's minimum PHP version increased to 5.5.9

Pursuant to the discussion at [policy] Require PHP 5.5, the minimum PHP version of Drupal 8 has been raised to 5.5.9, and this change will be included in the next Drupal 8 beta (8.0.0-beta13).

(PHP 5.5.9 was chosen because it is also the same minimum version as Ubuntu's LTS, which in turn influenced Symfony 3.0, Travis CI, etc.)

This is a future-proofing move which buys us a few things:

• Some nice language features and a built-in opcode cache.
• Compatibility with the latest versions of various external dependencies, including Guzzle 6 and the upcoming Symfony 3.0
• Better security for our end users, since PHP 5.4 will become end of life September 15, 2015 (most likely prior to Drupal 8's release).

We looked extensively into the adoption and hosting support of PHP 5.5 prior to making this move. While there is not widespread adoption of PHP 5.5 as of today, we nevertheless found that most hosts offer the option for PHP 5.5, due to PHP's security policy.