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

This makes xmpp-parsers usable on stable.

src/data_forms.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 std::str::FromStr;
 
 use minidom::{Element, IntoElements, IntoAttributeValue, ElementEmitter};
@@ -103,7 +103,7 @@ pub struct DataForm {
 }
 
 impl TryFrom<Element> for DataForm {
-    type Error = Error;
+    type Err = Error;
 
     fn try_from(elem: Element) -> Result<DataForm, Error> {
         if !elem.is("x", ns::DATA_FORMS) {