Pin aarch64 musl to 1.22.0

Apparently it fails on 1.23.0 for mysterious reasons

Author: alexcrichton

.travis.yml

@@ -53,7 +53,8 @@ matrix:
     - env: TARGET=arm-unknown-linux-musleabihf
     - env: TARGET=aarch64-unknown-linux-gnu
     - env: TARGET=aarch64-unknown-linux-musl
-      rust: beta
+      # FIXME(#856)
+      rust: 1.22.1
     - os: osx
       osx_image: xcode8.2
       env: TARGET=i386-apple-ios