Revert "[chore] Release 1.16.0 (#248)" (#249)

This reverts commit 72a8c2c.

Author: hiranya911

FirebaseAdmin/FirebaseAdmin/FirebaseAdmin.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <Version>1.16.0</Version>
+    <Version>1.15.0</Version>
     <TargetFrameworks>netstandard1.5;netstandard2.0;net45</TargetFrameworks>
     <TargetFrameworks Condition=" '$(OS)' != 'Windows_NT' ">netstandard1.5;netstandard2.0</TargetFrameworks>
     <LangVersion>latest</LangVersion>