README: Include CircleCI build status badge.
README: Include CircleCI build status badge.

For now, we still need the Travis badge, since Travis is where we test the 
production installation process.  But ideally, we'll end up removing that too.
hackerkid/zulip,tommyip/zulip,kou/zulip,tommyip/zulip,hackerkid/zulip,hackerkid/zulip,timabbott/zulip,hackerkid/zulip,andersk/zulip,hackerkid/zulip,timabbott/zulip,tommyip/zulip,shubhamdhama/zulip,rht/zulip,timabbott/zulip,punchagan/zulip,andersk/zulip,synicalsyntax/zulip,kou/zulip,zulip/zulip,shubhamdhama/zulip,dhcrzf/zulip,kou/zulip,synicalsyntax/zulip,brainwane/zulip,eeshangarg/zulip,showell/zulip,tommyip/zulip,jackrzhang/zulip,punchagan/zulip,zulip/zulip,punchagan/zulip,brainwane/zulip,showell/zulip,zulip/zulip,rishig/zulip,tommyip/zulip,zulip/zulip,brainwane/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,eeshangarg/zulip,rishig/zulip,eeshangarg/zulip,synicalsyntax/zulip,jackrzhang/zulip,punchagan/zulip,brainwane/zulip,punchagan/zulip,synicalsyntax/zulip,jackrzhang/zulip,andersk/zulip,brainwane/zulip,dhcrzf/zulip,zulip/zulip,rishig/zulip,kou/zulip,eeshangarg/zulip,synicalsyntax/zulip,rht/zulip,rishig/zulip,andersk/zulip,rht/zulip,shubhamdhama/zulip,eeshangarg/zulip,rht/zulip,dhcrzf/zulip,hackerkid/zulip,synicalsyntax/zulip,timabbott/zulip,rht/zulip,jackrzhang/zulip,andersk/zulip,dhcrzf/zulip,zulip/zulip,eeshangarg/zulip,rishig/zulip,dhcrzf/zulip,shubhamdhama/zulip,andersk/zulip,rishig/zulip,brainwane/zulip,shubhamdhama/zulip,rht/zulip,showell/zulip,eeshangarg/zulip,brainwane/zulip,kou/zulip,jackrzhang/zulip,rishig/zulip,jackrzhang/zulip,jackrzhang/zulip,timabbott/zulip,zulip/zulip,timabbott/zulip,kou/zulip,dhcrzf/zulip,timabbott/zulip,kou/zulip,showell/zulip,showell/zulip,tommyip/zulip,dhcrzf/zulip,hackerkid/zulip,shubhamdhama/zulip,tommyip/zulip,synicalsyntax/zulip,shubhamdhama/zulip,showell/zulip,andersk/zulip,showell/zulip