revert Rust toolchain to nightly-2019-07-15 to avoid LLVM bug on mips

Related PR: rust-lang/rust#62592

Author: wangrunji0408

.travis.yml

@@ -4,7 +4,7 @@ dist: bionic
 
 language: rust
 
-rust: nightly-2019-09-01
+rust: nightly-2019-07-15
 
 os:
   - linux

README.md

@@ -19,7 +19,7 @@ Supported architectures and boards:
 
 ### Environment
 
-* [Rust](https://www.rust-lang.org) toolchain at nightly-2019-09-01
+* [Rust](https://www.rust-lang.org) toolchain
 * Cargo tools: [cargo-xbuild](https://github.com/rust-osdev/cargo-xbuild)
 * [QEMU](https://www.qemu.org) >= 3.1.0
 * [musl-based GCC toolchains](https://musl.cc/)

rust-toolchain

@@ -1 +1 @@
-nightly-2019-09-01
+nightly-2019-07-15