Use a concrete release of singularity for integration test (#151)

Fixes #147

Running tests locally works:
```
✔ Volume "motionlarity_motion-singularity-volume"        Created   0.0s
 ✔ Container motionlarity-db-1                            Healthy  0.0s
 ✔ Container motionlarity-singularity_admin_init-1        Exited   0.0s
 ✔ Container motionlarity-singularity_content_provider-1  Started  0.0s
 ✔ Container motionlarity-singularity_deal_pusher-1       Started  0.0s
 ✔ Container motionlarity-singularity_deal_tracker-1      Started  0.0s
 ✔ Container motionlarity-singularity_dataset_worker-1    Started  0.0s
 ✔ Container motionlarity-singularity_api-1               Started  0.0s
✔ Container motionlarity-motion-1                        Started

Run Motion integration tests
MOTION_INTEGRATION_TEST='true' go test . -v --count=1
=== RUN   TestRoundTripPutStatusAndFullStorage
    integration_test.go:43: check existing deals
    integration_test.go:59: clearing boost publish queue
    integration_test.go:68: posting 8MB data into motion
    integration_test.go:75: {16901881-fc9b-48a2-ad19-07df40b8ac70}
    integration_test.go:80: requesting data back while still in source storage
    integration_test.go:92: waiting for singularity to make deals with boost
    integration_test.go:108: waiting for singularity to transfer data to boost
    integration_test.go:124: triggering data publish in boost
    integration_test.go:134: awaiting successful deal publishing
    integration_test.go:153: sleeping for 1 minute to allow cleanup worker to run, and indexing to complete
--- PASS: TestRoundTripPutStatusAndFullStorage (121.68s)
PASS
ok      github.com/filecoin-project/motion/integration/test     122.057s
```

integration/test/motionlarity/.env

@@ -1,4 +1,4 @@
-SINGULARITY_REF=:dummy
+SINGULARITY_REF=:v0.5.1
 LOTUS_TEST='true'
 LOTUS_API=http://lotus:1234/rpc/v1
 MOTION_PRICE_PER_GIB_EPOCH=0
@@ -8,4 +8,4 @@ MOTION_SINGULARITY_SCHEDULE_CRON='* * * * * *'
 MOTION_VERIFIED_DEAL=false
 MOTION_SINGULARITY_SCHEDULE_DEAL_NUMBER=25
 MARKET_DEAL_URL=
-MOTION_SINGULARITY_LOCAL_CLEANUP_INTERVAL=10s
+MOTION_SINGULARITY_LOCAL_CLEANUP_INTERVAL=10s