launcher.gcr.io references are obsolete in README.md

[balopat]

It looks like we should update launcher.gcr.io/google/jetty references to gcr.io/google-appengine/jetty

 gcloud beta container images list-tags launcher.gcr.io/google/jetty
DIGEST        TAGS    TIMESTAMP
7c37e49bfbbf  latest  2018-01-04T10:41:30
de73d24071dd          2017-12-21T17:14:05
d45ab119069b          2017-12-18T15:02:26

gcloud beta container images list-tags gcr.io/google-appengine/jetty
DIGEST        TAGS                           TIMESTAMP
dcfc5cf6401f  9,9.4-2018-03-02_21_33,latest  2018-03-02T16:35:38
a5fca05d4660  9.4-2018-02-28_19_28           2018-02-28T14:29:58
f9b8196492ff  9.4-2018-02-28_16_59           2018-02-28T12:00:57
7c37e49bfbbf  9.4-2018-01-04_15_39           2018-01-04T10:41:30
de73d24071dd  9.4-2017-12-21_22_12           2017-12-21T17:14:05
2e6e6303eeb2  9.4-2017-12-18_21_52           2017-12-18T16:54:09
d45ab119069b  9.4-2017-12-18_20_00           2017-12-18T15:02:26
76c30a70766a  9.4-2017-12-18_18_17           2017-12-18T13:19:19
772627942f69  9.4-2017-11-21_15_33           2017-11-21T10:35:21
55c2376084aa  9.4-2017-11-20_16_11           2017-11-20T11:13:20
```