Use docker for MIPS

Author: Alexander Polakov

.travis.yml

@@ -34,7 +34,7 @@ matrix:
       env: TARGET=arm-unknown-linux-gnueabihf
       rust: nightly
     - os: linux
-      env: TARGET=mips-unknown-linux-gnu
+      env: TARGET=mips-unknown-linux-gnu DOCKER=plhk/ubuntu-cross-mips:latest
       rust: nightly
     - os: linux
       env: TARGET=aarch64-unknown-linux-gnu