Don't include user generated MYMETA.json

I noticed that the file MYMETA.json has been commited to git. This file is [a user generated file](http://www.dagolden.com/index.php/479/mymeta-yml-in-the-wild/), and is regenerated when you do `perl Makefile.PL`. This causes diffs when doing unrelated development.

Another thing I should mention is that I also got warnings when doing `perl Makefile.PL` for missing files:

    $ perl Makefile.PL
    ...
    Warning: the following files are missing in your kit:
        META.yml
        README.md
    Please inform the author.
    ...

(This is not addressed in this pull request.)