Auto merge of #539 - posborne:new-ci/remove-parallelism, r=fiveop

ci: remove thread parallelism

Fixes #529, also refer to
#529 which first added this
for the old CI infrastructure.

Author: homu

.travis.yml

@@ -13,6 +13,7 @@ rust: stable
 env:
   global:
     - CRATE_NAME=nix
+    - RUST_TEST_THREADS=1
 
     # default job
     - TARGET=x86_64-unknown-linux-gnu