added travis status img into README
added travis status img into README

jhermann/nodeenv,chriskuehl/nodeenv,Milstein/nodeenv,asottile/nodeenv,twmt/nodeenv,Milstein/nodeenv,jamesblunt/nodeenv,simudream/nodeenv,twmt/nodeenv,jamesblunt/nodeenv,simudream/nodeenv,chriskuehl/nodeenv,asottile/nodeenv