Fixed build by locking Composer to v1 for mutation testing.

Bilge · Bilge · commit 831753752ecd · 2020-11-09T00:13:56.000Z
diff --git a/.travis.yml b/.travis.yml
@@ -22,6 +22,7 @@ cache:
     - vendor
 
 install:
+  - composer self-update --1
   - composer validate
   - composer --no-interaction update $DEPENDENCIES
 
