Not sure if this is a bug or what, but I don't see anything related to slime blocks being changed in any of the changelogs on the wiki. Slime blocks wouldn't even push players in the 15w38a snapshot, so I guess this was maybe mistakenly changed in the 15w38b snapshot.

Up to you guys weather this is a bug or not, but just saying, it breaks all slime block piston elevators...

Here is a video demonstrating that the height that slime blocks push you is different in 1.8.8 and 15w38a (Same effect in 15w42a): https://youtu.be/uGfJeYDp0Rk