Make build work with rustc 1.37 stable and RUSTC_BOOTSTRAP=1 (#3844)

* Make build work with rustc 1.37 stable and RUSTC_BOOTSTRAP=1

* Bump versions to run CI

substrate/core/sr-std/without_std.rs

@@ -14,7 +14,6 @@
 // You should have received a copy of the GNU General Public License
 // along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
 
-#[cfg(feature = "nightly")]
 #[doc(hidden)]
 pub extern crate alloc;