Fix supported Rust version in Travis config

jturner314 · jturner314 · commit ab47c1d37644 · 2018-09-04T01:24:38.000-04:00
`#[repr(transparent)]` was added in Rust 1.28.0.
diff --git a/.travis.yml b/.travis.yml
@@ -7,7 +7,7 @@ addons:
       - libssl-dev
 cache: cargo
 rust:
-  - 1.27.0
+  - 1.28.0
   - stable
   - beta
   - nightly
