Modernize std::error::Error impl, update dependencies

Author: mcasper

Cargo.lock

@@ -7,9 +7,10 @@ description = "A tool for cleaning old git branches."
 documentation = "https://github.com/mcasper/git-clean"
 homepage = "https://github.com/mcasper/git-clean"
 repository = "https://github.com/mcasper/git-clean"
+autotests = false
 
 [dependencies]
-clap = "2.23"
+clap = "2.33.1"
 
 [dev-dependencies]
 tempdir = "0.3"

Cargo.toml

@@ -14,31 +14,32 @@ pub enum Error {
 use self::Error::*;
 
 impl StdError for Error {
-    fn description(&self) -> &str {
+    fn source(&self) -> Option<&(dyn StdError + 'static)> {
         match *self {
-            Io(ref io_error) => io_error.description(),
-            ExitEarly => "",
+            Io(ref io_error) => Some(io_error),
+            _ => None,
+        }
+    }
+}
+
+impl Display for Error {
+    fn fmt(&self, f: &mut Formatter) -> Result<(), FmtError> {
+        match *self {
+            Io(ref io_error) => io_error.fmt(f),
+            ExitEarly => Ok(()),
             GitInstallationError => {
-                "Unable to execute 'git' on your machine, please make sure it's installed and on \
-                 your PATH"
+                write!(f, "Unable to execute 'git' on your machine, please make sure it's installed and on your PATH")
             }
             CurrentBranchInvalidError => {
-                "Please make sure to run git-clean from your base branch (defaults to master)."
+                write!(f, "Please make sure to run git-clean from your base branch (defaults to master).")
             }
             InvalidRemoteError => {
-                "That remote doesn't exist, please make sure to use a valid remote (defaults to \
-                 origin)."
+                write!(f, "That remote doesn't exist, please make sure to use a valid remote (defaults to origin).")
             }
         }
     }
 }
 
-impl Display for Error {
-    fn fmt(&self, f: &mut Formatter) -> Result<(), FmtError> {
-        self.description().fmt(f)
-    }
-}
-
 impl From<IoError> for Error {
     fn from(error: IoError) -> Error {
         Io(error)