Change the trivial cast lints to allow by default

src/librand/distributions/range.rs

@@ -10,8 +10,6 @@
 
 //! Generating numbers between two others.
 
-#![allow(trivial_numeric_casts)]
-
 // this is surprisingly complicated to be both generic & correct
 
 use core::prelude::{PartialOrd};

src/librand/isaac.rs

@@ -447,7 +447,6 @@ impl Rng for Isaac64Rng {
 
     #[inline]
     fn next_u64(&mut self) -> u64 {
-        #![allow(trivial_numeric_casts)]
         if self.cnt == 0 {
             // make some more numbers
             self.isaac64();