pin to an older nightly

cf. rust-lang/cargo#3819

Author: Jorge Aparicio

.travis.yml

@@ -24,7 +24,7 @@ matrix:
 
     # Nightly, for testing
     - env: TARGET=x86_64-unknown-linux-gnu
-      rust: nightly
+      rust: nightly-2017-03-04
 
 install:
   - sh ci/install.sh