readme: Remove Travis badge.
readme: Remove Travis badge.

Travis hasn't been our primary CI system for a long time, and flakes,
and the badges look duplicate with the CircleCI one, so we should just
remove it.

See discussion at
https://chat.zulip.org/#narrow/stream/101-design/topic/github.20badges.

kou/zulip,zulip/zulip,punchagan/zulip,eeshangarg/zulip,synicalsyntax/zulip,brainwane/zulip,zulip/zulip,shubhamdhama/zulip,timabbott/zulip,hackerkid/zulip,punchagan/zulip,andersk/zulip,showell/zulip,rht/zulip,synicalsyntax/zulip,shubhamdhama/zulip,hackerkid/zulip,synicalsyntax/zulip,showell/zulip,synicalsyntax/zulip,timabbott/zulip,eeshangarg/zulip,synicalsyntax/zulip,kou/zulip,rht/zulip,andersk/zulip,kou/zulip,shubhamdhama/zulip,rht/zulip,timabbott/zulip,shubhamdhama/zulip,brainwane/zulip,kou/zulip,brainwane/zulip,zulip/zulip,rht/zulip,andersk/zulip,punchagan/zulip,eeshangarg/zulip,eeshangarg/zulip,hackerkid/zulip,synicalsyntax/zulip,zulip/zulip,shubhamdhama/zulip,brainwane/zulip,shubhamdhama/zulip,zulip/zulip,showell/zulip,rht/zulip,showell/zulip,rht/zulip,hackerkid/zulip,showell/zulip,eeshangarg/zulip,timabbott/zulip,kou/zulip,timabbott/zulip,andersk/zulip,andersk/zulip,andersk/zulip,timabbott/zulip,zulip/zulip,timabbott/zulip,punchagan/zulip,punchagan/zulip,punchagan/zulip,brainwane/zulip,rht/zulip,kou/zulip,synicalsyntax/zulip,andersk/zulip,showell/zulip,hackerkid/zulip,hackerkid/zulip,hackerkid/zulip,shubhamdhama/zulip,kou/zulip,eeshangarg/zulip,showell/zulip,brainwane/zulip,punchagan/zulip,eeshangarg/zulip,zulip/zulip,brainwane/zulip