Bump version, upgrade bootstrap

This commit updates the version number to 1.17.0 as we're not on that version of
the nightly compiler, and at the same time this updates src/stage0.txt to
bootstrap from freshly minted beta compiler and beta Cargo.

src/libcore/iter/range.rs

@@ -242,7 +242,6 @@ step_impl_signed!(i64);
 // assume here that it is less than 64-bits.
 #[cfg(not(target_pointer_width = "64"))]
 step_impl_no_between!(u64 i64);
-#[cfg(not(stage0))]
 step_impl_no_between!(u128 i128);
 
 /// An adapter for stepping range iterators by a custom amount.