Update forgotten exposition client links in Make.
Update forgotten exposition client links in Make.

The links in the Makefile, which Travis uses, were out-of-date in
terms of fetching the Go client library.

q3k/prometheus,AlekSi/prometheus,tomwilkie/prometheus,gesellix/prometheus,JavierZunzunegui/prometheus,jbdalido/prometheus,mrwacky42/prometheus,prometheus/prometheus,fabxc/prometheus,wndhydrnt/prometheus,rajthilakmca/prometheus,prometheus/prometheus,thachmai/prometheus,alileza/prometheus,trigrass2/prometheus,hustbill/prometheus,wrouesnel/prometheus,gottwald/prometheus,PrFalken/prometheus,jbdalido/prometheus,mtanda/prometheus,tomwilkie/prometheus,GoogleCloudPlatform/prometheus,dln/prometheus,prasincs/prometheus,dmilstein/prometheus,dsymonds/prometheus,AlekSi/prometheus,Gouthamve/prometheus,justintung/prometheus,adamron1/prometheus,prasincs/prometheus,yanghongkjxy/prometheus,robbiet480/prometheus,hustbill/prometheus,agaoglu/prometheus,slok/prometheus,eliothedeman/prometheus,agaoglu/prometheus,jbdalido/prometheus,kzittritsch/prometheus,dmilstein/prometheus,thachmai/prometheus,smyrman/prometheus,q3k/prometheus,nicr9/prometheus,cit-lab/prometheus,trigrass2/prometheus,agaoglu/prometheus,laushinka/prometheus,mrwacky42/prometheus,gcnonato/prometheus,laushinka/prometheus,gcnonato/prometheus,wrouesnel/prometheus,prometheus/prometheus,kangwoo/prometheus,Dieterbe/prometheus,gesellix/prometheus,smyrman/prometheus,Gouthamve/prometheus,agaoglu/prometheus,fabric8io/prometheus,tattsun/prometheus,brancz/prometheus,AlekSi/prometheus,prometheus/prometheus,AlekSi/prometheus,keegancsmith/prometheus,RobertKielty/prometheus,nicr9/prometheus,kzittritsch/prometheus,laushinka/prometheus,jimmidyson/prometheus,thachmai/prometheus,abhishekamralkar/prometheus,wxdublin/prometheus,amorken/prometheus,adamron1/prometheus,Conorbro/prometheus,trigrass2/prometheus,kzittritsch/prometheus,wndhydrnt/prometheus,fabric8io/prometheus,eliothedeman/prometheus,dpatel06/prometheus,brancz/prometheus,sudhi-vm/prometheus,JavierZunzunegui/prometheus,tattsun/prometheus,brian-brazil/prometheus,dalanlan/prometheus,mtanda/prometheus,iksaif/prometheus,flier/prometheus,sudhi-vm/prometheus,justintung/prometheus,dln/prometheus,hustbill/prometheus,dpatel06/prometheus,smyrman/prometheus,wxdublin/prometheus,slok/prometheus,Dieterbe/prometheus,kangwoo/prometheus,jbdalido/prometheus,keegancsmith/prometheus,fabric8io/prometheus,dmilstein/prometheus,tattsun/prometheus,robbiet480/prometheus,RobertKielty/prometheus,sudhi-vm/prometheus,dsymonds/prometheus,alileza/prometheus,alileza/prometheus,rajthilakmca/prometheus,yanghongkjxy/prometheus,abhishekamralkar/prometheus,laushinka/prometheus,jmcfarlane/prometheus,alileza/prometheus,keegancsmith/prometheus,Colstuwjx/prometheus,thachmai/prometheus,brian-brazil/prometheus,PrFalken/prometheus,jmcfarlane/prometheus,datajet-io/prometheus,dalanlan/prometheus,kzittritsch/prometheus,adamron1/prometheus,Stackdriver/stackdriver-prometheus,fabxc/prometheus,dmilstein/prometheus,msiebuhr/prometheus,iksaif/prometheus,dpatel06/prometheus,PrFalken/prometheus,Stackdriver/stackdriver-prometheus,gcnonato/prometheus,Conorbro/prometheus,justintung/prometheus,bluecmd/prometheus,Gouthamve/prometheus,wxdublin/prometheus,RobertKielty/prometheus,Colstuwjx/prometheus,wrouesnel/prometheus,Stackdriver/stackdriver-prometheus,prometheus/prometheus,bluecmd/prometheus,gottwald/prometheus,SaltedMan/prometheus-annotated,smyrman/prometheus,gottwald/prometheus,hustbill/prometheus,dsymonds/prometheus,brancz/prometheus,trigrass2/prometheus,datajet-io/prometheus,JavierZunzunegui/prometheus,Stackdriver/stackdriver-prometheus,dln/prometheus,ton31337/prometheus,dalanlan/prometheus,ton31337/prometheus,Gouthamve/prometheus,flier/prometheus,Colstuwjx/prometheus,robbiet480/prometheus,fabxc/prometheus,dsymonds/prometheus,tomwilkie/prometheus,q3k/prometheus,flier/prometheus,GoogleCloudPlatform/prometheus,datajet-io/prometheus,tomwilkie/prometheus,msiebuhr/prometheus,fabric8io/prometheus,robbiet480/prometheus,JavierZunzunegui/prometheus,cit-lab/prometheus,abhishekamralkar/prometheus,keegancsmith/prometheus,slok/prometheus,amorken/prometheus,msiebuhr/prometheus,Colstuwjx/prometheus,prasincs/prometheus,mtanda/prometheus,keegancsmith/prometheus,adamron1/prometheus,dln/prometheus,rajthilakmca/prometheus,PrFalken/prometheus,yanghongkjxy/prometheus,jimmidyson/prometheus,wrouesnel/prometheus,GoogleCloudPlatform/prometheus,prasincs/prometheus,gesellix/prometheus,jmcfarlane/prometheus,gcnonato/prometheus,Dieterbe/prometheus,cit-lab/prometheus,amorken/prometheus,yanghongkjxy/prometheus,jmcfarlane/prometheus,brancz/prometheus,tattsun/prometheus,eliothedeman/prometheus,q3k/prometheus,eliothedeman/prometheus,amorken/prometheus,mrwacky42/prometheus,cit-lab/prometheus,mtanda/prometheus,fabxc/prometheus,SaltedMan/prometheus-annotated,ton31337/prometheus,bluecmd/prometheus,ton31337/prometheus,gottwald/prometheus,nicr9/prometheus,bluecmd/prometheus,Conorbro/prometheus,Conorbro/prometheus,msiebuhr/prometheus,datajet-io/prometheus,brian-brazil/prometheus,iksaif/prometheus,wndhydrnt/prometheus,gesellix/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,sudhi-vm/prometheus,RobertKielty/prometheus,GoogleCloudPlatform/prometheus,justintung/prometheus,slok/prometheus,SaltedMan/prometheus-annotated,kangwoo/prometheus,abhishekamralkar/prometheus,jimmidyson/prometheus,rajthilakmca/prometheus,alileza/prometheus,SaltedMan/prometheus-annotated,iksaif/prometheus