Modify checkstyle to accept new copyright header.
Modify checkstyle to accept new copyright header.

As discussed in https://github.com/GoogleCloudPlatform/DataflowJavaSDK/pull/10
With external contributors the copyright header should be:
Copyright (C) 2015 The Google Cloud Dataflow Authors

Old copyright header is still accepted.

haonaturel/DataflowJavaSDK,mxm/incubator-beam,jasonkuster/incubator-beam,haocafes/DataflowJavaSDK,iemejia/incubator-beam,apache/beam,joshualitt/DataflowJavaSDK,vikkyrk/incubator-beam,haonature888/DataflowJavaSDK,chamikaramj/beam,RyanSkraba/beam,robertwb/incubator-beam,Priyalc/Gcloud,elibixby/DataflowJavaSDK,rangadi/incubator-beam,markflyhigh/incubator-beam,staslev/beam,jbonofre/beam,vikkyrk/incubator-beam,amitsela/beam,robertwb/incubator-beam,charlesccychen/beam,charlesccychen/beam,rangadi/beam,manuzhang/beam,charlesccychen/incubator-beam,peihe/incubator-beam,vemmaverve/DataflowJavaSDK,prabeesh/DataflowJavaSDK,jbonofre/beam,tweise/incubator-beam,robertwb/incubator-beam,joshualitt/DataflowJavaSDK,chamikaramj/beam,charlesccychen/beam,tweise/incubator-beam,nevillelyh/DataflowJavaSDK,haonature/DataflowJavaSDK,charlesccychen/beam,smarthi/DataflowJavaSDK,elibixby/DataflowJavaSDK,chamikaramj/beam,sureshjosh/DataflowJavaSDK,nagachika/DataflowJavaSDK,lukecwik/incubator-beam,jasonkuster/beam,prabeesh/DataflowJavaSDK,robertwb/incubator-beam,haonaturel/DataflowJavaSDK,tgroh/beam,charlesccychen/beam,amitsela/incubator-beam,lukecwik/incubator-beam,tgroh/incubator-beam,chamikaramj/beam,apache/beam,dhananjaypatkar/DataflowJavaSDK,markflyhigh/incubator-beam,lukecwik/incubator-beam,tyagihas/DataflowJavaSDK,Test-Betta-Inc/musical-umbrella,manuzhang/beam,markflyhigh/incubator-beam,dhalperi/beam,dhalperi/beam,tgroh/incubator-beam,yk5/beam,vikkyrk/incubator-beam,charlesccychen/incubator-beam,joshualitt/incubator-beam,markflyhigh/incubator-beam,ravwojdyla/incubator-beam,RyanSkraba/beam,robertwb/incubator-beam,ravwojdyla/incubator-beam,peihe/DataflowJavaSDK,rangadi/beam,tgroh/beam,jasonkuster/beam,wangyum/beam,dhalperi/incubator-beam,ravwojdyla/incubator-beam,Priyalc/Gcloud,wtanaka/beam,jasonkuster/beam,rangadi/beam,sammcveety/DataflowJavaSDK,rangadi/beam,amarouni/incubator-beam,lukecwik/incubator-beam,robertwb/incubator-beam,amitsela/incubator-beam,springml/DataflowJavaSDK,PieterDM/DataflowJavaSDK,RyanSkraba/beam,jbonofre/incubator-beam,josauder/AOP_incubator_beam,tyagihas/DataflowJavaSDK,wangyum/beam,apache/beam,chamikaramj/beam,joshualitt/incubator-beam,sammcveety/incubator-beam,amitsela/beam,dhananjaypatkar/DataflowJavaSDK,lukecwik/incubator-beam,staslev/beam,chamikaramj/MyDataflowJavaSDK,wangyum/beam,tweise/beam,staslev/incubator-beam,mxm/incubator-beam,RyanSkraba/beam,tgroh/beam,Test-Betta-Inc/musical-umbrella,wangyum/beam,robertwb/incubator-beam,haonature/DataflowJavaSDK,lukecwik/incubator-beam,dhalperi/incubator-beam,xsm110/Apache-Beam,vemmaverve/DataflowJavaSDK,haonature888/DataflowJavaSDK,chamikaramj/beam,markflyhigh/incubator-beam,chamikaramj/beam,markflyhigh/incubator-beam,tomwhite/DataflowJavaSDK,apache/beam,vikaspalkar/DataflowJavaSDK,lukecwik/incubator-beam,eljefe6a/incubator-beam,tomwhite/DataflowJavaSDK,eljefe6a/incubator-beam,apache/beam,jbonofre/beam,springml/DataflowJavaSDK,xsm110/Apache-Beam,chamikaramj/beam,peihe/DataflowJavaSDK,twcctz500000/DataflowJavaSDK,PieterDM/DataflowJavaSDK,xsm110/Apache-Beam,wtanaka/beam,markflyhigh/incubator-beam,peihe/incubator-beam,yk5/beam,chamikaramj/beam,josauder/AOP_incubator_beam,apache/beam,sammcveety/incubator-beam,lukecwik/incubator-beam,rangadi/incubator-beam,jasonkuster/incubator-beam,amarouni/incubator-beam,rangadi/beam,chamikaramj/MyDataflowJavaSDK,charlesccychen/beam,robertwb/incubator-beam,lukecwik/incubator-beam,apache/beam,sureshjosh/DataflowJavaSDK,staslev/incubator-beam,RyanSkraba/beam,chamikaramj/beam,twcctz500000/DataflowJavaSDK,jbonofre/beam,lukecwik/incubator-beam,tgroh/beam,apache/beam,sammcveety/DataflowJavaSDK,manuzhang/incubator-beam,staslev/beam,charlesccychen/incubator-beam,yk5/beam,vikaspalkar/DataflowJavaSDK,ahartley39/DataflowJavaSDK,tweise/beam,manuzhang/beam,RyanSkraba/beam,jbonofre/incubator-beam,amitsela/beam,rangadi/beam,charlesccychen/beam,chamikaramj/incubator-beam,apache/beam,smarthi/DataflowJavaSDK,dhalperi/beam,apache/beam,rangadi/beam,robertwb/incubator-beam,wtanaka/beam,RyanSkraba/beam,haocafes/DataflowJavaSDK,apache/beam,sammcveety/incubator-beam,rangadi/incubator-beam,robertwb/incubator-beam,iemejia/incubator-beam,peihe/incubator-beam,manuzhang/incubator-beam,eljefe6a/incubator-beam,chamikaramj/incubator-beam,ahartley39/DataflowJavaSDK,nagachika/DataflowJavaSDK