pin to an older nightly until nightly Cargo is fixed

Jorge Aparicio · Jorge Aparicio · commit c7e37ce7a947 · 2017-03-14T14:35:26.000-05:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,7 +1,7 @@
 cache: cargo
 dist: trusty
 language: rust
-rust: nightly
+rust: nightly-2017-03-04
 services: docker
 sudo: required
 
