Fix compiler warnings from Rustc bump

src/lib.rs

@@ -332,7 +332,7 @@ impl StdError for Error {
         }
     }
 
-    fn cause(&self) -> Option<&StdError> {
+    fn cause(&self) -> Option<&dyn StdError> {
         match *self {
             Error::Io(ref err) => Some(err),
             Error::Mustache(ref err) => Some(err),

src/print/mod.rs

@@ -27,7 +27,7 @@ use std::io::{self, Write};
 use std::path::PathBuf;
 use toml::Value;
 
-fn destination(output: Option<&PathBuf>) -> Result<Box<Write>> {
+fn destination(output: Option<&PathBuf>) -> Result<Box<dyn Write>> {
     if let Some(ref output) = output {
         trace!("An output path has been explicity specified");
         let f = File::create(output)?;

src/sign.rs

@@ -231,7 +231,7 @@ impl Execution {
     fn msi(&self) -> Result<PathBuf> {
         if let Some(ref i) = self.input {
             trace!("The path to an installer to sign has been explicitly set");
-            let mut msi = PathBuf::from(i);
+            let msi = PathBuf::from(i);
             if msi.exists() {
                 trace!("The installer exists");
                 Ok(msi)