Raise minimum Rust version to 1.32

The crypto_api_chachapoly uses two new features introduced in 1.32:

- Self struct constructors
- u64::to_le_bytes()

.travis.yml

@@ -1,6 +1,6 @@
 language: rust
 rust:
-  - 1.31.0
+  - 1.32.0
 
 cache: cargo