Mention dropped `.pre-scrollable` class in migration guide.
Mention dropped `.pre-scrollable` class in migration guide.

GerHobbelt/bootstrap,coliff/bootstrap,peterblazejewicz/bootstrap,GerHobbelt/bootstrap,tjkohli/bootstrap,twbs/bootstrap,inway/bootstrap,m5o/bootstrap,inway/bootstrap,zalog/bootstrap,zalog/bootstrap,m5o/bootstrap,coliff/bootstrap,zalog/bootstrap,peterblazejewicz/bootstrap,GerHobbelt/bootstrap,tjkohli/bootstrap,peterblazejewicz/bootstrap,inway/bootstrap,twbs/bootstrap