Have you [Rebuild] the solution after this change ? Also, i would suggest to copy all default bunch of dll's to your project's [bin] folder, then [rebuild] the solution. you can get all dll's from the installed folder of your sitefinity.i.e. i've a sitefinity 5.0 installation. so i would get all untouched dll's from below path.
C:\Program Files (x86)\Telerik\Sitefinity 5.0\_EmptyProject\bin