Make the compiler field static

According to nunit/nunit#947, `ValueSourceAttributes` expects the referenced field to be `static` in NUnit 3.

src/GitVersionTask.Tests/AssemblyInfoBuilderTests.cs

@@ -53,7 +53,7 @@ public Compilation Compile(string assemblyInfoText)
         public string ApprovedSubFolder { get { return Path.Combine("Approved", "VisualBasic"); } }
     }
 
-    private readonly ICompiler[] compilers = new ICompiler[]
+    private static readonly ICompiler[] compilers = new ICompiler[]
     {
         new CSharpCompiler(),
         new VisualBasicCompiler()