Exclude environment.yml from sdist.

Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>

MANIFEST.in

diff --git a/MANIFEST.in b/MANIFEST.in
index e278e4f..712f2bc 100644
--- a/MANIFEST.in
+++ b/MANIFEST.in
@@ -1,4 +1,5 @@
 exclude .readthedocs.yml
+exclude environment.yml
 exclude tox.ini
 exclude *.swp
 prune conf