travis: drop beta Rust target

I don't think we've ever seen a build failure on this test target so
it seems like a waste of resources to continuously build it.

Author: mgeisler

.travis.yml

@@ -3,7 +3,6 @@ language: rust
 rust:
   - 1.31.0 # Rust 2018
   - stable
-  - beta
   - nightly
 
 cache: cargo