Bump minimum Rust version to 1.34.2

Tests have been failing for a while due to lack of support for 1.28 in
rayon and its dependencies. This bumps the minimum version to 1.34 to
match the `image` crate.

Author: kevinmehall

.travis.yml

@@ -1,6 +1,6 @@
 language: rust
 rust:
-  - 1.28.0
+  - 1.34.2
   - stable
   - beta
   - nightly

src/marker.rs

@@ -132,7 +132,6 @@ impl Marker {
             0xFD => Some(JPGn(13)),
             0xFE => Some(COM),
             0xFF => None, // Fill byte
-            _ => unreachable!(),
         }
     }
 }