Switch to stable rust to work around rust-lang/rust#47466.

Chris Pearce · Chris Pearce · commit 3abb0b7ff7d8 · 2018-01-16T10:34:49.000+13:00
diff --git a/.travis.yml b/.travis.yml
@@ -2,7 +2,7 @@ dist: trusty
 sudo: 9000
 language: rust
 rust:
-  - nightly
+  - stable
 os:
   - linux
   - osx
