Use high-quality icon for nuspecs

nuget/nunit.runners.xamarin.nuspec

@@ -17,7 +17,7 @@ Supported Xamarin platforms:
         <language>en-US</language>
         <licenseUrl>http://nunit.org/nuget/nunit3-license.txt</licenseUrl>
         <projectUrl>https://github.com/nunit/nunit.xamarin</projectUrl>
-        <iconUrl>http://nunit.org/nuget/nunitv3_32x32.png</iconUrl>
+        <iconUrl>https://cdn.rawgit.com/nunit/resources/master/images/icon/nunit_256.png</iconUrl>
         <requireLicenseAcceptance>false</requireLicenseAcceptance>
         <copyright>Copyright (c) 2015 Charlie Poole</copyright>