Resolve ignored_unit_patterns pedantic clippy lint

    warning: matching over `()` is more explicit
      --> src/date.rs:25:30
       |
    25 |     try_parse(iter).map_err(|_| {
       |                              ^ help: use `()` instead of `_`: `()`
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ignored_unit_patterns
       = note: `-W clippy::ignored-unit-patterns` implied by `-W clippy::pedantic`

    warning: matching over `()` is more explicit
      --> src/release.rs:13:30
       |
    13 |     try_parse(iter).map_err(|_| Error::group(paren, "expected rustc release number, like 1.31"))
       |                              ^ help: use `()` instead of `_`: `()`
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ignored_unit_patterns

src/date.rs

@@ -22,7 +22,7 @@ impl Display for Date {
 }
 
 pub fn parse(paren: Group, iter: Iter) -> Result<Date> {
-    try_parse(iter).map_err(|_| {
+    try_parse(iter).map_err(|()| {
         let msg = format!("expected nightly date, like {}", time::today());
         Error::group(paren, msg)
     })

src/release.rs

@@ -10,7 +10,7 @@ pub struct Release {
 }
 
 pub fn parse(paren: Group, iter: Iter) -> Result<Release> {
-    try_parse(iter).map_err(|_| Error::group(paren, "expected rustc release number, like 1.31"))
+    try_parse(iter).map_err(|()| Error::group(paren, "expected rustc release number, like 1.31"))
 }
 
 fn try_parse(iter: Iter) -> Result<Release, ()> {