ci: remove thread parallelism

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

Signed-off-by: Paul Osborne <[email protected]>

Author: Paul Osborne

.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