Drop support for rust 1.13 and 1.14

TheServerAsterisk · TheServerAsterisk · commit 80de43e551b7 · 2017-05-22T08:24:31.000-06:00
diff --git a/.travis.yml b/.travis.yml
@@ -7,8 +7,8 @@ rust:
   - nightly
 
   # The two most recent stable releases before "stable"
-  - 1.13.0
-  - 1.14.0
+  - 1.15.0
+  - 1.16.0
 
 matrix:
   allow_failures:
