Fix dependency on microformats/tests

[aaronpk]

The 0.3.1 release relies on finding the microformats/tests repo by its git URL rather than looking it up from packagist. Instead this should reference the packagist version of the library, and only as a dev dependency. I'll fix this as soon as microformats/tests is published on packagist.

[aaronpk]

mf2/tests is now on packagist! https://packagist.org/packages/mf2/tests

[jkphl]

Yay! So now there's only another php-mf2 release missing. Maybe you could consider #121 to go into that as well?

[aaronpk]

This has been resolved in the v0.3.2 release!