Update CompilationUtil.cs (#140)

roslyn folder is a Directory not a File

Author: coffeecop

src/Microsoft.CodeDom.Providers.DotNetCompilerPlatform/Util/CompilationUtil.cs

@@ -208,7 +208,7 @@ internal static string CompilerDefaultPath()
             string compilerFullPath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, webPath);
 
             // Then appdomain base
-            if (!File.Exists(compilerFullPath))
+            if (!Directory.Exists(compilerFullPath))
                 compilerFullPath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, appPath);
 
             return compilerFullPath;