Fix a test that fails on Travis CI.

- 'trap' command should use the signal name without the "SIG" prefix.
diff --git a/tests/executor/test_mysos_task_runner.py b/tests/executor/test_mysos_task_runner.py
index 287ed2f..311d97d 100644
--- a/tests/executor/test_mysos_task_runner.py
+++ b/tests/executor/test_mysos_task_runner.py
@@ -186,7 +186,7 @@
             "Command 'exit 1' returned non-zero exit status 1")
 
   def test_stop_interminable(self):
-    cmd = """trap "echo Trapped SIGTERM!" SIGTERM
+    cmd = """trap "echo Trapped SIGTERM!" TERM
 while :
 do
   sleep 60