Replace (string) casting with (array) in version::getPrerelease().
Merge branch 'master' of https://github.com/Pheromone/php-semver

Closes #32: Replace (string) casting with (array) in
version::getPrerelease().

* 'master' of https://github.com/Pheromone/php-semver:
  Replace (string) casting with (array) in version::getPrerelease().
Test all getters on the version object

Prevent stuff like in #32