Update WebSocket4Net dependency

[zaccharles]

This PR is to update WebSocket4Net from 0.14.1 to 0.15.2, which is the latest version. I've been running this fork for about a week now and haven't had any issues. Below is the changelog between these two versions.

0.15.2

• Upgraded SuperSocket.ClientEngine to support NTLM;

0.15.1

• fixing the bug with queued packets not being sent over TLS/SSL;
• added a new security option AllowCertificateChainErrors;
• fixed one unhandled exception which may be thrown when the connection drops;

0.15.0

• fixed the wrong upgrade format in response;
• added null check for websocket instance in SendCloseHandshake;
• moved the header item "Host" to the first position to comply rfc7230;
• fixed a fragmenting issue when do handshake;
• fixed the user-agent typo in the head;
• fixed the exception which may be thrown when try to send close handshake;
• when validating the verbLine, do not require that the optional description parameter be present;
• fixed the signing issue;
• removed the support for the target framework WP and Xamarin.

[damdo]

Closing due to earlier update. Thank you @zaccharles