Upgrade sockjs to 0.3.19

[fwielstra]

I was running auditjs on my project which came across one vulnerability in the version of sockjs used by webpack-dev-server (issue, fix, which was resolved in the 0.3.19 release (from about a year ago). I've searched and couldn't find this issue having been raised yet.

PR incoming.

[SpaceK33z]

Looks to have been fixed a while ago, so closing.