Remove some older Rusts from CI

Some dependencies don't build on 1.11 and 1.10, so we'll have to scrap
them unfortunately.

Author: mhallin

.travis.yml

@@ -6,9 +6,7 @@ rust:
   - beta
   - nightly
 
-  # The five most recent stable releases before "stable"
-  - 1.10.0
-  - 1.11.0
+  # The three most recent stable releases before "stable"
   - 1.12.1
   - 1.13.0
   - 1.14.0