update contributing.md - remove /var/run/ add libffi-dev
update contributing.md - remove /var/run/ add libffi-dev

amyxchen/openhtf,fahhem/openhtf,google/openhtf,jettisonjoe/openhtf,fahhem/openhtf,grybmadsci/openhtf,ShaperTools/openhtf,amyxchen/openhtf,ShaperTools/openhtf,google/openhtf,fahhem/openhtf,jettisonjoe/openhtf,ShaperTools/openhtf,ShaperTools/openhtf,jettisonjoe/openhtf,jettisonjoe/openhtf,ShaperTools/openhtf,google/openhtf,fahhem/openhtf,grybmadsci/openhtf,grybmadsci/openhtf,google/openhtf,grybmadsci/openhtf