Drop dependency on try_from.

This bumps the minimum supported stable Rust version to 1.34.

The TryFrom and TryInto traits are still reexported to not break the
API, but these reexports are deprecated and will be removed in a future
release.

examples/generate-caps.rs

@@ -4,10 +4,10 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this
 // file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
+use std::convert::TryFrom;
 use std::io::{self, Read};
 use std::env;
 use xmpp_parsers::{
-    TryFrom,
     Element,
     disco::DiscoInfoResult,
     caps::{Caps, compute_disco as compute_disco_caps, hash_caps},