Add leif to requirements.txt
Add leif to requirements.txt

Actually it should probably be entered as a real dependency in setup.py,
but right now i am not sure. This suffices for travis for now.

mathstuf/vdirsyncer,tribut/vdirsyncer,untitaker/vdirsyncer,untitaker/vdirsyncer,untitaker/vdirsyncer,mathstuf/vdirsyncer,credativUK/vdirsyncer,tribut/vdirsyncer,credativUK/vdirsyncer,hobarrera/vdirsyncer,hobarrera/vdirsyncer