use short-array syntax, and get_called_class instead of static::class

[abacaphiliac]

use short-array syntax and get_called_class instead of static::class for php54 and php53 support.

[abacaphiliac]

@rquadling this should fix the master build: https://travis-ci.org/beberlei/assert/builds/188943175

[rquadling]

I got called away before I could see the completion of the tests.

Thank you.

[abacaphiliac]

@rquadling no problem at all. i'm here to help : )

this is a super helpful lib. thank you for your effort and support.

[rquadling]

I'm just a collaborator. I use it as part of my work, so supporting it means I give back.