Fix nightly travis build failures

* Currently nightly will ICE when run with `-C link-dead-code`:
see rust-lang/rust#47309 for more info
* Workaround is to run kcov on stable instead until a fix is in place

Author: ipetkov

.travis.yml

@@ -15,13 +15,14 @@ addons:
 matrix:
   include:
     - rust: nightly
+
+    - rust: stable # Running kcov on stable due to an ICE on nightly: https://github.com/rust-lang/rust/issues/47309
       env:
         - RUSTFLAGS="-C link-dead-code" # Enable better code coverage at the cost of binary size
       after_success:
         - ./ci/install-kcov.sh && ./ci/coverage.sh
 
     - rust: beta
-    - rust: stable
     - rust: 1.20.0
     - os: osx
       rust: stable