Update nuspec to only install dependencies for target framework.

Author: Cory Thompson

WebPush.nuspec

@@ -9,15 +9,33 @@
     <projectUrl>https://github.com/coryjthompson/web-push-csharp/</projectUrl>
     <requireLicenseAcceptance>false</requireLicenseAcceptance>
     <description>Web Push library for C#</description>
-    <copyright>Copyright 2016</copyright>
+    <copyright>Copyright 2017</copyright>
     <tags>web push notifications vapid</tags>
-    <dependencies>
-       <dependency id="BouncyCastle" version="1.8.1"/>
-       <dependency id="Newtonsoft.Json" version="6.0.1"/>
-       <dependency id="Microsoft.Bcl" version="1.1.10" />
-       <dependency id="Microsoft.Bcl.Async" version="1.0.168" />
-       <dependency id="Microsoft.Bcl.Build" version="1.0.14" />
-       <dependency id="Microsoft.Net.Http" version="2.2.29" />
+    <dependencies>	
+        <group targetFramework="netcoreapp1.0">
+            <dependency id="BouncyCastle.NetCore" version="1.8.1.3"/>
+            <dependency id="Newtonsoft.Json" version="10.0.1"/>
+        </group>
+        <group targetFramework="netcoreapp1.1">
+            <dependency id="BouncyCastle.NetCore" version="1.8.1.3"/>
+            <dependency id="Newtonsoft.Json" version="10.0.1"/>
+        </group>
+        <group targetFramework="netstandard1.3">
+            <dependency id="BouncyCastle.NetCore" version="1.8.1.3"/>
+            <dependency id="Newtonsoft.Json" version="10.0.1"/>
+        </group>
+
+        <group targetFramework="net45">
+            <dependency id="BouncyCastle" version="1.8.1"/>
+	    <dependency id="Microsoft.Net.Http" version="2.2.29" />
+            <dependency id="Newtonsoft.Json" version="10.0.1"/>
+        </group>
+
+        <group targetFramework="net46">
+            <dependency id="BouncyCastle" version="1.8.1"/>
+            <dependency id="Microsoft.Net.Http" version="2.2.29" />
+            <dependency id="Newtonsoft.Json" version="10.0.1"/>
+        </group>
     </dependencies>
   </metadata>
 </package>