Don't depend on NUnit, no code uses it in this package

nuspec/MvvmCross.Tests.nuspec

@@ -13,11 +13,10 @@ This package contains the 'Test Helpers' for MvvmCross
 		<dependencies>
 			<dependency id="MvvmCross.Core" version="[$version$]" />
 			<dependency id="MvvmCross.Platform" version="[$version$]" />
-			<dependency id="NUnit" version="3.6.0" />
 		</dependencies>
 	</metadata>
 	<files>
 		<!-- full -->
 		<file src="..\bin\Release\Mvx\Portable\MvvmCross.Test.Core.*" target="lib\portable45-net45+win8+wp8+wpa81" />
 	</files>
-</package>
+</package>