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/libserialize/json.rs

@@ -211,8 +211,6 @@ use std::string;
 use std::{char, f64, fmt, str};
 use std;
 
-use rustc_i128::{i128, u128};
-
 use Encodable;
 
 /// Represents a json value