Update version info to 2.3.1

Build/MQTTnet.nuspec

@@ -2,7 +2,7 @@
 <package >
   <metadata>
     <id>MQTTnet</id>
-    <version>2.3.0</version>
+    <version>2.3.1</version>
     <authors>Christian Kratky</authors>
     <owners>Christian Kratky</owners>
     <licenseUrl>https://github.com/chkr1011/MQTTnet/blob/master/LICENSE</licenseUrl>
@@ -10,13 +10,7 @@
     <iconUrl>https://raw.githubusercontent.com/chkr1011/MQTTnet/master/Images/Logo_128x128.png</iconUrl>
     <requireLicenseAcceptance>false</requireLicenseAcceptance>
     <description>MQTTnet is a .NET library for MQTT based communication. It provides a MQTT client and a MQTT server (broker).</description>
-    <releaseNotes>* [Client] Added support for web socket communication channel (thanks to nowakpiotr)
-* [Core] Huge performance optimizations (thanks to JanEggers)
-* [Client] Fixed an issue when calling _DisconnectAsync_ and the connection is already closed.
-* [Core] Added several new interfaces
-* [Core] Added a factory for creation of application messages
-* [Core] Fixed issues in connection handling
-* [Client] Fixed issues with subscribing/unsubscribing (breaking change!)
+    <releaseNotes>* [Server] Fixed issue when accepting a new connection (UWP only) (Thanks to haeberle)
     </releaseNotes>
     <copyright>Copyright Christian Kratky 2016-2017</copyright>
     <tags>MQTT Message Queue Telemetry Transport MQTTClient MQTTServer Server MQTTBroker Broker NETStandard IoT InternetOfThings Messaging Hardware Arduino Sensor Actuator M2M</tags>