Increase minimum Rust version for unwrap_or_default

Author: palfrey

.travis.yml

@@ -5,7 +5,8 @@ rust:
 - nightly
 # Oldest supported version for all features.
 # Use of https://github.com/rust-lang/rfcs/pull/16
-- 1.14.0
+# and https://github.com/rust-lang/rust/issues/37516
+- 1.16.0
 # Oldest supported version as dependency, with no features, tests, or examples.
 - 1.10.0