Drop PHP 5.3 Support In 2.1

[vlucas]

Drop PHP 5.3 Support In 2.1

[vlucas]

There is no real need to (which is the only reason I didn't in 2.0.0), but 5.3 is deprecated and no longer receiving updates, so it does make sense to drop it from 2.1.0.

[xjchengo]

If dropping php 5.3 support, major version should bump to follow semantic versioning.

[GrahamCampbell]

That depends if we're following semver or not.

[huglester]

Probably best was to drop it's support when jumping to 2.0. but now it's too late :(

[GrahamCampbell]

That's what I wanted. :(

[GrahamCampbell]

but 5.3 is deprecated and no longer

That was true when we released 2.0.0 too @vlucas.

[vlucas]

We can't do this if we want to follow semver, so we're stuck with it now. Fortunately, this library doesn't really do anything fancy, so it's fine.