Add Java 7 requirement for building gerrit
Add Java 7 requirement for building gerrit

We don't support building with Java 8 yet, add the information to
build documentation.

Change-Id: I88b28b23e7c1275a44a8c8fa6082edae5df3333a

GerritCodeReview/gerrit,joshuawilson/merrit,hdost/gerrit,renchaorevee/gerrit,TonyChai24/test,Overruler/gerrit,gcoders/gerrit,thinkernel/gerrit,anminhsu/gerrit,Overruler/gerrit,jackminicloud/test,Distrotech/gerrit,MerritCR/merrit,hdost/gerrit,joshuawilson/merrit,thinkernel/gerrit,dwhipstock/gerrit,netroby/gerrit,WANdisco/gerrit,gcoders/gerrit,pkdevbox/gerrit,Seinlin/gerrit,joshuawilson/merrit,thinkernel/gerrit,netroby/gerrit,hdost/gerrit,qtproject/qtqa-gerrit,quyixia/gerrit,netroby/gerrit,Overruler/gerrit,joshuawilson/merrit,dwhipstock/gerrit,joshuawilson/merrit,hdost/gerrit,thesamet/gerrit,supriyantomaftuh/gerrit,jackminicloud/test,TonyChai24/test,quyixia/gerrit,TonyChai24/test,pkdevbox/gerrit,Seinlin/gerrit,Distrotech/gerrit,renchaorevee/gerrit,dwhipstock/gerrit,Distrotech/gerrit,anminhsu/gerrit,renchaorevee/gerrit,WANdisco/gerrit,TonyChai24/test,TonyChai24/test,pkdevbox/gerrit,thesamet/gerrit,gcoders/gerrit,gcoders/gerrit,WANdisco/gerrit,supriyantomaftuh/gerrit,joshuawilson/merrit,thesamet/gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,quyixia/gerrit,netroby/gerrit,quyixia/gerrit,renchaorevee/gerrit,jackminicloud/test,GerritCodeReview/gerrit,gerrit-review/gerrit,gerrit-review/gerrit,quyixia/gerrit,GerritCodeReview/gerrit,gerrit-review/gerrit,Seinlin/gerrit,pkdevbox/gerrit,Overruler/gerrit,GerritCodeReview/gerrit,thesamet/gerrit,quyixia/gerrit,WANdisco/gerrit,gcoders/gerrit,thinkernel/gerrit,jackminicloud/test,MerritCR/merrit,renchaorevee/gerrit,thinkernel/gerrit,thinkernel/gerrit,thesamet/gerrit,Distrotech/gerrit,supriyantomaftuh/gerrit,TonyChai24/test,GerritCodeReview/gerrit,GerritCodeReview/gerrit,supriyantomaftuh/gerrit,thinkernel/gerrit,pkdevbox/gerrit,joshuawilson/merrit,thesamet/gerrit,pkdevbox/gerrit,Seinlin/gerrit,Seinlin/gerrit,Overruler/gerrit,MerritCR/merrit,Distrotech/gerrit,gcoders/gerrit,MerritCR/merrit,GerritCodeReview/gerrit,netroby/gerrit,Distrotech/gerrit,anminhsu/gerrit,anminhsu/gerrit,Seinlin/gerrit,qtproject/qtqa-gerrit,hdost/gerrit,quyixia/gerrit,renchaorevee/gerrit,jackminicloud/test,Distrotech/gerrit,gerrit-review/gerrit,gerrit-review/gerrit,hdost/gerrit,supriyantomaftuh/gerrit,anminhsu/gerrit,MerritCR/merrit,supriyantomaftuh/gerrit,joshuawilson/merrit,renchaorevee/gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,anminhsu/gerrit,qtproject/qtqa-gerrit,dwhipstock/gerrit,anminhsu/gerrit,dwhipstock/gerrit,gerrit-review/gerrit,qtproject/qtqa-gerrit,supriyantomaftuh/gerrit,WANdisco/gerrit,netroby/gerrit,MerritCR/merrit,dwhipstock/gerrit,jackminicloud/test,netroby/gerrit,TonyChai24/test,Seinlin/gerrit,hdost/gerrit,MerritCR/merrit,pkdevbox/gerrit,gerrit-review/gerrit,thesamet/gerrit,Overruler/gerrit,jackminicloud/test,gcoders/gerrit,MerritCR/merrit,GerritCodeReview/gerrit,dwhipstock/gerrit