Use TryFrom from the try_from crate, instead of the unstable feature.

This makes xmpp-parsers usable on stable.

src/stanza_id.rs

@@ -4,7 +4,7 @@
 // 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 try_from::TryFrom;
 
 use minidom::Element;
 use jid::Jid;
@@ -25,7 +25,7 @@ pub enum StanzaId {
 }
 
 impl TryFrom<Element> for StanzaId {
-    type Error = Error;
+    type Err = Error;
 
     fn try_from(elem: Element) -> Result<StanzaId, Error> {
         let is_stanza_id = elem.is("stanza-id", ns::SID);