Add meta_test_fixtures to the MANIFEST

[blopker]

Via email:

I'm using yamale for some time and eventually tried
to make debian package for it. I use the following command line:

$python setup.py --command-packages=stdeb.command bdist_deb

It results me errors, because it doesn't copy files from
yamale/tests/meta_test_fixtures to deb_dist/ directory as other needed
files. I made the following change and it began to work:

$ git diff
diff --git a/MANIFEST.in b/MANIFEST.in
index 91b9824..440b84f 100644
--- a/MANIFEST.in
+++ b/MANIFEST.in
@@ -1,2 +1,3 @@
   include *.md LICENSE
   include yamale/tests/fixtures/*.yaml
+include yamale/tests/meta_test_fixtures/*.yaml

I'm using f5444a3602e07871c4700efe6aaf75c51addb384 revision of yamale
sources.

I hope this will be useful for you.