Merge #187

187: Allow nightly to fail in Travis r=therealprof a=korken89

With nightly being a bit skiddish, lets allow nightly to fail.

Anyone against? @rust-embedded/cortex-m 

- [x] @adamgreig 
- [x] @korken89
- [x] @thejpster
- [ ] @ithinuel 
- [x] @therealprof 

Co-authored-by: Emil Fresk <[email protected]>

Author: bors[bot]

cortex-m-rt/.travis.yml

@@ -1,6 +1,8 @@
 language: rust
 
 matrix:
+  allow_failures:
+    - rust: nightly
   include:
     - env: TARGET=x86_64-unknown-linux-gnu
       rust: stable