Fix Flow failure on CircleCI due to the number of workers

https://github.com/flowtype/flow-bin/issues/138#issuecomment-448416874
https://github.com/flowtype/flow-bin/issues/138#issuecomment-450367472

> This will stop flow from trying to consume too many virtual CPUs (which CircleCI doesn't actually provide), letting it run to completion without first running out of memory and killing its sub processes.

The log output in CircleCI was:
https://circleci.com/gh/bvaughn/react-devtools-experimental/201
```
#!/bin/bash -eo pipefail
yarn flow
yarn run v1.10.1
$ /home/circleci/repo/node_modules/.bin/flow
Launching Flow server for /home/circleci/repo
Spawned flow server (pid=259)
Logs will go to /tmp/flow/zShomezScirclecizSrepo.log
Monitor logs will go to /tmp/flow/zShomezScirclecizSrepo.monitor_log
Launching Flow server for /home/circleci/repo
Spawned flow server (pid=361)
Logs will go to /tmp/flow/zShomezScirclecizSrepo.log
Monitor logs will go to /tmp/flow/zShomezScirclecizSrepo.monitor_log
Launching Flow server for /home/circleci/repo
Spawned flow server (pid=464)
Logs will go to /tmp/flow/zShomezScirclecizSrepo.log
Monitor logs will go to /tmp/flow/zShomezScirclecizSrepo.monitor_log
Launching Flow server for /home/circleci/repo
Spawned flow server (pid=567)
Logs will go to /tmp/flow/zShomezScirclecizSrepo.log
Monitor logs will go to /tmp/flow/zShomezScirclecizSrepo.monitor_log
Lost connection to the flow server (0 retries remaining): -Out of retries, exiting!
error Command failed with exit code 7.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
Exited with code 1
```

.flowconfig

@@ -25,6 +25,7 @@ shells/dev/build/*
 [lints]
 
 [options]
+server.max_workers=4
 esproposal.class_instance_fields=enable
 suppress_comment=\\(.\\|\n\\)*\\$FlowFixMe
 suppress_comment=\\(.\\|\n\\)*\\$FlowIssue