Update Apitools to version 0.5.6
Update Apitools to version 0.5.6

This brings in the fix to https://github.com/google/apitools/pull/136
needed for the BigQuery reader.

jbonofre/beam,amarouni/incubator-beam,apache/beam,lukecwik/incubator-beam,dhalperi/beam,apache/beam,mxm/incubator-beam,apache/beam,rangadi/beam,markflyhigh/incubator-beam,jasonkuster/incubator-beam,rangadi/incubator-beam,peihe/incubator-beam,RyanSkraba/beam,dhalperi/incubator-beam,jbonofre/beam,lukecwik/incubator-beam,markflyhigh/incubator-beam,jbonofre/beam,chamikaramj/beam,peihe/incubator-beam,wtanaka/beam,iemejia/incubator-beam,manuzhang/beam,chamikaramj/beam,peihe/incubator-beam,staslev/beam,charlesccychen/incubator-beam,chamikaramj/incubator-beam,rangadi/beam,staslev/beam,rangadi/beam,charlesccychen/beam,staslev/beam,robertwb/incubator-beam,RyanSkraba/beam,vikkyrk/incubator-beam,robertwb/incubator-beam,xsm110/Apache-Beam,robertwb/incubator-beam,rangadi/beam,wangyum/beam,chamikaramj/beam,RyanSkraba/beam,apache/beam,robertwb/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,vikkyrk/incubator-beam,tgroh/beam,mxm/incubator-beam,amarouni/incubator-beam,charlesccychen/beam,chamikaramj/beam,robertwb/incubator-beam,lukecwik/incubator-beam,lukecwik/incubator-beam,yk5/beam,apache/beam,robertwb/incubator-beam,dhalperi/beam,tgroh/incubator-beam,staslev/incubator-beam,charlesccychen/beam,jasonkuster/beam,eljefe6a/incubator-beam,markflyhigh/incubator-beam,ravwojdyla/incubator-beam,iemejia/incubator-beam,apache/beam,apache/beam,xsm110/Apache-Beam,chamikaramj/beam,RyanSkraba/beam,wangyum/beam,sammcveety/incubator-beam,dhalperi/incubator-beam,manuzhang/beam,markflyhigh/incubator-beam,wtanaka/beam,jbonofre/incubator-beam,charlesccychen/beam,chamikaramj/beam,yk5/beam,apache/beam,rangadi/beam,sammcveety/incubator-beam,markflyhigh/incubator-beam,yk5/beam,RyanSkraba/beam,apache/beam,jbonofre/beam,tgroh/beam,jasonkuster/beam,apache/beam,chamikaramj/beam,amitsela/incubator-beam,manuzhang/incubator-beam,wangyum/beam,charlesccychen/beam,rangadi/incubator-beam,apache/beam,rangadi/beam,amitsela/beam,tgroh/incubator-beam,tgroh/beam,charlesccychen/beam,lukecwik/incubator-beam,markflyhigh/incubator-beam,manuzhang/incubator-beam,wangyum/beam,amitsela/beam,amitsela/beam,ravwojdyla/incubator-beam,robertwb/incubator-beam,jasonkuster/beam,eljefe6a/incubator-beam,chamikaramj/incubator-beam,lukecwik/incubator-beam,lukecwik/incubator-beam,robertwb/incubator-beam,lukecwik/incubator-beam,charlesccychen/beam,RyanSkraba/beam,lukecwik/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,jbonofre/incubator-beam,xsm110/Apache-Beam,sammcveety/incubator-beam,chamikaramj/beam,tgroh/beam,amitsela/incubator-beam,rangadi/incubator-beam,charlesccychen/incubator-beam,wtanaka/beam,RyanSkraba/beam,vikkyrk/incubator-beam,ravwojdyla/incubator-beam,lukecwik/incubator-beam,markflyhigh/incubator-beam,dhalperi/beam,rangadi/beam,eljefe6a/incubator-beam,staslev/incubator-beam,jasonkuster/incubator-beam,charlesccychen/incubator-beam,manuzhang/beam