Composer extension will now pull the PHP version out of `composer.json`
or `composer.lock` (`composer.json` is checked first).  If it's not
specified, it defaults to the version set by the build pack (i.e.
options.json).

The following syntax is supported for version numbers.

  - 5.3.* -> latest 5.4.x release (5.3, not supported)
  - >=5.3 -> latest 5.4.x release (5.3, not supported)
  - 5.4.* -> latest 5.4.x release
  - >=5.4 -> latest 5.4.x release
  - 5.5.* -> latest 5.5.x release
  - >=5.5 -> latest 5.5.x release
  - 5.4.31 (or some other specific release number) -> exact version
  - 5.5.15 (or some other specific release number) -> exact version