fixes scrolling with trackpoint

[toniz4]

By scrolling "harder" with my thinkpad x230 trackpoint, using libinput drivers, it will report a button 6 event. Possibly is a bug with libinput, when i was using the synaptics drivers in arch i don't remember having this problem. example xev output:

ButtonRelease event, serial 32, synthetic NO, window 0x2000001,
    root 0x139, subw 0x0, time 15265257, (82,90), root:(673,382),
    state 0x800, button 4, same_screen YES

ButtonPress event, serial 32, synthetic NO, window 0x2000001,
    root 0x139, subw 0x0, time 15265257, (82,90), root:(673,382),
    state 0x0, button 4, same_screen YES

ButtonRelease event, serial 32, synthetic NO, window 0x2000001,
    root 0x139, subw 0x0, time 15265257, (82,90), root:(673,382),
    state 0x800, button 4, same_screen YES

ButtonPress event, serial 32, synthetic NO, window 0x2000001,
    root 0x139, subw 0x0, time 15265282, (82,90), root:(673,382),
    state 0x0, button 6, same_screen YES

ButtonRelease event, serial 32, synthetic NO, window 0x2000001,
    root 0x139, subw 0x0, time 15265282, (82,90), root:(673,382),
    state 0x0, button 6, same_screen YES

ButtonPress event, serial 32, synthetic NO, window 0x2000001,
    root 0x139, subw 0x0, time 15265282, (82,90), root:(673,382),
    state 0x0, button 4, same_screen YES

ButtonRelease event, serial 32, synthetic NO, window 0x2000001,
    root 0x139, subw 0x0, time 15265282, (82,90), root:(673,382),
    state 0x800, button 4, same_screen YES

ButtonPress event, serial 32, synthetic NO, window 0x2000001,
    root 0x139, subw 0x0, time 15265282, (82,90), root:(673,382),
    state 0x0, button 4, same_screen YES

ButtonRelease event, serial 32, synthetic NO, window 0x2000001,
    root 0x139, subw 0x0, time 15265282, (82,90), root:(673,382),
    state 0x800, button 4, same_screen YES

the button 6 events comes when i'm scrolling faster.
fixes #58

[Airblader]

Thanks for the PR. However, this sounds like a bandaid fix. Button 6 is not a scrolling event, so I think you should rather file a bug with libinput for this. I don't think I'm willing to merge this since it's factually just not correct.

[toniz4]

Understandable, you might need to add xsltproc to the travis installed packages so the build will not fail