thumb targets are not yet on stable

Author: japaric

.travis.yml

@@ -3,8 +3,8 @@ language: rust
 matrix:
   include:
     - env: TARGET=x86_64-unknown-linux-gnu
-    - env: TARGET=thumbv6m-none-eabi
-    - env: TARGET=thumbv7m-none-eabi
+    # - env: TARGET=thumbv6m-none-eabi
+    # - env: TARGET=thumbv7m-none-eabi
     - env: TARGET=thumbv6m-none-eabi
       rust: nightly
     - env: TARGET=thumbv7m-none-eabi