Use const for compat with rustc 1.13-16

Was using static but that's only supported as of rustc 1.17 (rust
these older versions. Also continue using the copious explicit 'static
lifetimes for the same compatibility, despite the clippy lint.

src/datetime.rs

@@ -287,7 +287,7 @@ impl<Tz: TimeZone> DateTime<Tz> where Tz::Offset: fmt::Display {
         use format::Fixed::*;
         use format::Fixed::Nanosecond;
 
-        static PREFIX: &[Item<'static>] = &[
+        const PREFIX: &'static [Item<'static>] = &[
             Numeric(Year, Zero),
             Literal("-"),
             Numeric(Month, Zero),