Fix deprecated CLOB type declaration in SQL Server platform

According to the [SQL Server documentation](http://technet.microsoft.com/en-gb/library/ms187993.aspx) the `text` type declaration is deprecated and will be removed in the future. It is encouraged to use `varchar(max)` instead. This has been applied to `SQLServerPlatform::getClobTypeDeclarationSQL()` by this PR.
The `varchar(max)` declaration is BC for all SQL Server versions supported by Doctrine btw.