fix nuget version core lib

Author: Ellerbach

nanoFramework.Azure.Devices.Client.FullyManaged.nuspec

@@ -20,7 +20,7 @@ This is an SDK for Azure IoT Hub using a MQTT broker interface. This is Independ
 Use the nanoFrameowrk.Azure.Devices.Client if you want to use native wifi and MQTT broker.</description>
     <tags>nanoFramework C# csharp netmf netnf Microsoft.Azure.Devices Microsoft.Azure.Devices.Client nanoFramework.Azure.Devices.Client.FullyManaged</tags>
     <dependencies>
-      <dependency id="nanoFramework.CoreLibrary" version="1.15.6" />
+      <dependency id="nanoFramework.CoreLibrary" version="1.15.5" />
       <dependency id="nanoFramework.Json" version="2.2.160" />
       <dependency id="nanoFramework.M2Mqtt.Core" version="5.1.158" />
       <dependency id="nanoFramework.Runtime.Native" version="1.7.1" />

nanoFramework.Azure.Devices.Client.nuspec

@@ -19,7 +19,7 @@
 This is an SDK for Azure IoT Hub using MQTT broker.</description>
     <tags>nanoFramework C# csharp netmf netnf Microsoft.Azure.Devices nanoFramework.Azure.Devices</tags>
     <dependencies>
-      <dependency id="nanoFramework.CoreLibrary" version="1.15.6" />
+      <dependency id="nanoFramework.CoreLibrary" version="1.15.5" />
       <dependency id="nanoFramework.Json" version="2.2.160" />
       <dependency id="nanoFramework.M2Mqtt" version="5.1.158" />
       <dependency id="nanoFramework.System.Net.Http" version="1.5.99" />