Set number of concurrent test threads to 1
diff --git a/.travis.yml b/.travis.yml
index fcbd1ab..b63b4a0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -16,4 +16,4 @@
   - cargo fmt --all -- --check
   - cargo check --benches --bins --examples --tests
   - CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUNNER="valgrind --leak-check=full --error-exitcode=1"
-    cargo test
+    cargo test -- --test-threads=1