meta: bump metapackage to new target frameworks

we now target net46, netstd1.3, and netstd2.0... this bumps from the
previous versions of net45, netstd1.1, netstd1.3

src/Discord.Net/Discord.Net.nuspec

@@ -13,21 +13,21 @@
     <requireLicenseAcceptance>false</requireLicenseAcceptance>
     <iconUrl>https://github.com/RogueException/Discord.Net/raw/dev/docs/marketing/logo/PackageLogo.png</iconUrl>
     <dependencies>
-        <group targetFramework="net45">
+        <group targetFramework="net46">
             <dependency id="Discord.Net.Core" version="2.0.0-beta2$suffix$" />
             <dependency id="Discord.Net.Rest" version="2.0.0-beta2$suffix$" />
             <dependency id="Discord.Net.WebSocket" version="2.0.0-beta2$suffix$" />
             <dependency id="Discord.Net.Commands" version="2.0.0-beta2$suffix$" />
             <dependency id="Discord.Net.Webhook" version="2.0.0-beta2$suffix$" />
         </group>
-        <group targetFramework="netstandard1.1">
+        <group targetFramework="netstandard1.3">
             <dependency id="Discord.Net.Core" version="2.0.0-beta2$suffix$" />
             <dependency id="Discord.Net.Rest" version="2.0.0-beta2$suffix$" />
             <dependency id="Discord.Net.WebSocket" version="2.0.0-beta2$suffix$" />
             <dependency id="Discord.Net.Commands" version="2.0.0-beta2$suffix$" />
             <dependency id="Discord.Net.Webhook" version="2.0.0-beta2$suffix$" />
         </group>
-        <group targetFramework="netstandard1.3">
+        <group targetFramework="netstandard2.0">
             <dependency id="Discord.Net.Core" version="2.0.0-beta2$suffix$" />
             <dependency id="Discord.Net.Rest" version="2.0.0-beta2$suffix$" />
             <dependency id="Discord.Net.WebSocket" version="2.0.0-beta2$suffix$" />