use older nightly for the x86_64 linux target

.travis.yml

@@ -3,7 +3,8 @@ language: rust
 matrix:
   include:
     - env: TARGET=x86_64-unknown-linux-gnu
-      rust: nightly
+      # work around problem about 'every suggestion should have at least one span'
+      rust: nightly-2017-11-21
 
     - env: TARGET=thumbv6m-none-eabi
       rust: nightly