Specifying the Mobile NuGet only on platforms

- not supported in PCL
 - specify version number for Xamairn.Forms

Source/OxyPlot.Xamarin.Forms/OxyPlot.Xamarin.Forms.nuspec

@@ -13,10 +13,24 @@
     <requireLicenseAcceptance>false</requireLicenseAcceptance>
     <tags>plotting plot charting chart xamarin forms android windows phone ios</tags>
     <dependencies>
+	  <group targetFramework="MonoTouch">
+        <dependency id="OxyPlot.Mobile" version="[0.0.0]"/>
+      </group>
+      <group targetFramework="Xamarin.iOS10">
+        <dependency id="OxyPlot.Mobile" version="[0.0.0]"/>
+      </group>
+      <group targetFramework="MonoAndroid">
+        <dependency id="OxyPlot.Mobile" version="[0.0.0]"/>
+      </group>
+      <group targetFramework="windowsphone8">
+        <dependency id="OxyPlot.Mobile" version="[0.0.0]"/>
+      </group>
+      <group targetFramework="portable-windows8+wpa81">
+        <dependency id="OxyPlot.Mobile" version="[0.0.0]"/>
+      </group>
       <group>
         <dependency id="OxyPlot.Core" version="[0.0.0]"/>
-        <dependency id="OxyPlot.Mobile" version="[0.0.0]"/>
-        <dependency id="Xamarin.Forms"/>
+        <dependency id="Xamarin.Forms" version="1.3.3.6323" />
       </group>
     </dependencies>
   </metadata>