Add race detection on Travis CI.

thanks to @ppknap for the suggestion
diff --git a/.travis.yml b/.travis.yml
index 1b5151f..7098876 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,11 +2,14 @@
 language: go
 
 go:
-  - 1.5.1
+  - 1.5.2
 
 before_script:
   - go get -u github.com/golang/lint/golint
 
+script:
+  - go test -v --race ./...
+
 after_script:
   - test -z "$(gofmt -s -l -w . | tee /dev/stderr)"
   - test -z "$(golint ./...     | tee /dev/stderr)"