Consider upgrading vulnerable version of java-websocket

[lalvarezguillen]

There's a security advisory against java-websocket<=1.4.1: https://nvd.nist.gov/vuln/detail/CVE-2020-11050

ably-java is using java-websocket:1.4.0. I couldn't find an issue mentioning this CVE, so I'm creating the issue to document and consider upgrading.

┆Issue is synchronized with this Jira Bug by Unito

[QuintinWillison]

Relates to #749.

[deanna-seta]

Closed by #787