Add Travis build status to README
Add Travis build status to README

gaowenbin/govalidator,spid37/govalidator,nathj07/govalidator,asaskevich/govalidator,mgiaccone/govalidator,yh453926638/govalidator,hvnsweeting/govalidator,mondough/govalidator,annismckenzie/govalidator,xiqingping/govalidator,tsaikd/govalidator,rakhnin/govalidator,rounds/govalidator,majidgolshadi/govalidator,zj8487/govalidator,Dadie/govalidator