Update JS flow dependencies to fix security issues

[wing328]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

To address the following security alert:

Remediation
Upgrade mem to version 4.0.0 or later. For example:

"dependencies": {
  "mem": ">=4.0.0"
}
or…
"devDependencies": {
  "mem": ">=4.0.0"
}
Always verify the validity and compatibility of suggestions with your codebase.

[wing328]

cc @CodeNinjai (2017/07) @frol (2017/07) @cliffano (2017/07)

also cc @ jaypea (the author of the Javascript (Flow types) generator)