Travis CI: set test timeout to 5 minutes

Travis CI has a no-output-timeout of 10 minutes.
Set a lower test timeout so we get proper backtraces
on a hung test.

Actual test runtime should be about 1 minute.

Change-Id: I3fb40d13fb5bf9efc5e96f3551733205a7502481

.travis.yml

@@ -25,5 +25,8 @@ install:
   - go get -t ./...
   - go get -t -race ./...
 
+# Travis CI has a no-output-timeout of 10 minutes.
+# Set a lower test timeout so we get proper backtraces
+# on a hung test.
 script:
-  - go test -v ./...
+  - go test -timeout 5m -v ./...