travis: update/simplify the support rust versions a bit

Since this isn't a library, not bumping major version as we
can be a bit more flexible with semver.

Signed-off-by: Paul Osborne <[email protected]>

Author: Paul Osborne

.travis.yml

@@ -2,10 +2,8 @@ language: rust
 sudo: false
 
 rust:
-  - 1.13.0
   - stable
-  - beta
-  - nightly
+  - 1.16.0
 
 script:
   - cargo test