sra_client.py/test: change launch kit docker image

Previously these teses were using 0xorg/launch-kit-ci, but that was a
one-off thing created just for CI, back before there was a regularly
maintained docker image of Launch Kit.

Changed to use 0xorg/launch-kit-backend since it's regularly
maintained/updated.

Because the -backend image is using a different Linux distribution
(Alpine), the commands used to wait for ganache startup also had to
change.

The tag used, 74bcc39, is provisional due to the pending Issue at
0xProject/0x-launch-kit-backend#73 .  When
that issue is resolved, the tag suffix on the imag name should be
removed.

.circleci/config.yml

@@ -182,12 +182,13 @@ jobs:
         docker:
             - image: circleci/python
             - image: 0xorg/ganache-cli:2.2.2
-            - image: 0xorg/launch-kit-ci
+            - image: 0xorg/launch-kit-backend:74bcc39
               environment:
                   RPC_URL: http://localhost:8545
                   NETWORK_ID: 50
                   WHITELIST_ALL_TOKENS: True
-              command: bash -c "until curl -sfd'{\"method\":\"net_listening\"}' http://localhost:8545 | grep true; do continue; done; forever ts/lib/index.js"
+              command: |
+                  sh -c "until printf 'POST /\r\nContent-Length: 26\r\n\r\n{\"method\":\"net_listening\"}' | nc localhost 8545 | grep true; do continue; done; node_modules/.bin/forever ts/lib/index.js"
         steps:
             - checkout
             - run: sudo chown -R circleci:circleci /usr/local/bin

python-packages/sra_client/test/relayer/docker-compose.yml

@@ -6,7 +6,7 @@ services:
         ports:
             - "8545:8545"
     launchkit:
-        image: "0xorg/launch-kit-ci"
+        image: "0xorg/launch-kit-backend:74bcc39"
         depends_on:
             - ganache
         ports:
@@ -16,4 +16,5 @@ services:
             - NETWORK_ID=50
             - RPC_URL=http://localhost:8545
             - WHITELIST_ALL_TOKENS=True
-        command: bash -c "until curl -sfd'{\"method\":\"net_listening\"}' http://localhost:8545 | grep true; do continue; done; forever ts/lib/index.js"
+        command: |
+            sh -c "until printf 'POST /\r\nContent-Length: 26\r\n\r\n{\"method\":\"net_listening\"}' | nc localhost 8545 | grep true; do continue; done; node_modules/.bin/forever ts/lib/index.js"