Bump version to 2.8.0

Reflects signalapp/libsignal-protocol-java@a3cff01

Author: golf1052

libsignal-protocol-dotnet/libsignal-protocol-dotnet.csproj

@@ -3,7 +3,7 @@
   <PropertyGroup>
     <TargetFramework>netstandard1.4</TargetFramework>
     <GeneratePackageOnBuild>false</GeneratePackageOnBuild>
-    <Version>2.7.1</Version>
+    <Version>2.8.0</Version>
     <Authors>golf1052, langboost, Trolldemorted</Authors>
     <PackageRequireLicenseAcceptance>True</PackageRequireLicenseAcceptance>
     <PackageLicenseExpression>GPL-3.0-only</PackageLicenseExpression>