Include the fernet vectors in the sdist

We include the other vectors, so consistency!
diff --git a/MANIFEST.in b/MANIFEST.in
index 67e9da1..c5ca035 100644
--- a/MANIFEST.in
+++ b/MANIFEST.in
@@ -4,4 +4,5 @@
 include README.rst
 
 recursive-include tests *.py
+recursive-include tests/vectors *
 recursive-include tests/hazmat/primitives/vectors *