Drop Ruby 2.4 until it's fixed
diff --git a/.travis.yml b/.travis.yml
index 27e1dd1..24c6234 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,6 +3,5 @@
   - 2.1.5
   - 2.2.7
   - 2.3.4
-  - 2.4.1
 script:
   - rake