[doc] Add Travis build status image
[doc] Add Travis build status image

mylikes/director,tanzhihang/director,vitalsource/director,maxogden/director,meleslilijing/director,kiritoe/director,pocketjoso/director,noxryan/director,cpg1111/director,flatiron/director,webx32/director,GerHobbelt/director,andradedev/director,stonegithubs/director,mylikes/director,fadeit/director,pandoraui/director,andradedev/director,meleslilijing/director,GerHobbelt/director,sheldonzf/director,apeace/director,webx32/director,lzhengms/director,ks3dev/director,kiritoe/director,cpg1111/director,stonegithubs/director,ks3dev/director,fadeit/director,tanzhihang/director,vitalsource/director,pandoraui/director,sheldonzf/director,lzhengms/director,pocketjoso/director