Drop support for old compilers
diff --git a/.travis.yml b/.travis.yml
index 8f17d02..f987dc2 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,7 +3,6 @@
 
 matrix:
   include:
-    - rust: 1.15.0
     - rust: 1.31.0
     - rust: stable
     - rust: beta