No Entity Framework provider found for the ADO.NET provider with invariant name 'System.Data.SqlClient'

Our solution just got upgraded from VS2010 to VS2013. In VS2010 application is running fine however I am getting this error "No Entity Framework provider found for the ADO.NET provider with invariant name 'System.Data.SqlClient'. Make sure the provider is registered in the 'entityFramework' section of the application config file. See http://go.microsoft.com/fwlink/?LinkId=260882 for more information." when I am trying to run the application. I tried to install entity framework through nuget package. Also I tried registering the entity framework assembly in the app config file but still I am getting this error.  My application is targeting to .net 4.5.1 and I am using EF 6.0. Please help< ...