Deterministic task ID test failure on Mac.

The following fails on Mac OS.

```
cd ray/python/ray/core
bash ../../../src/common/test/run_tests.sh
```

It fails with

```
* Suite task_tests:
.F
FAIL deterministic_ids_test: TaskID_equal(TaskSpec_task_id(spec1), TaskSpec_task_id(spec2)) (/Users/rkn/Workspace/ray/src/common/test/task_tests.cc:73)
.
3 tests - 2 passed, 1 failed, 0 skipped (178 ticks, 0.000 sec)
```