(fix) add Sync for error cause [#194]

Signed-off-by: Ning Sun <sunng@about.me>

src/error.rs

@@ -13,7 +13,7 @@ pub struct RenderError {
    pub template_name: Option<String>,
    pub line_no: Option<usize>,
    pub column_no: Option<usize>,
    cause: Option<Box<Error + Send>>,
    cause: Option<Box<Error + Send + Sync>>,
}

impl fmt::Display for RenderError {
@@ -69,7 +69,7 @@ impl RenderError {

    pub fn with<E>(cause: E) -> RenderError
    where
        E: Error + Send + 'static,
        E: Error + Send + Sync + 'static,
    {
        let mut e = RenderError::new(cause.description());
        e.cause = Some(Box::new(cause));