travis: Cache the built emoji across builds in Travis CI.
travis: Cache the built emoji across builds in Travis CI.

Apparently, when we implemented the emoji cache for development, we
failed to properly register its directory for Travis CI.

Commit message rewritten by tabbott.

jrowan/zulip,amanharitsh123/zulip,synicalsyntax/zulip,rishig/zulip,rht/zulip,showell/zulip,zulip/zulip,brainwane/zulip,showell/zulip,jackrzhang/zulip,hackerkid/zulip,punchagan/zulip,brainwane/zulip,rishig/zulip,jackrzhang/zulip,andersk/zulip,rishig/zulip,punchagan/zulip,synicalsyntax/zulip,Galexrt/zulip,timabbott/zulip,andersk/zulip,brainwane/zulip,hackerkid/zulip,eeshangarg/zulip,tommyip/zulip,tommyip/zulip,shubhamdhama/zulip,vaidap/zulip,jackrzhang/zulip,timabbott/zulip,dhcrzf/zulip,jrowan/zulip,punchagan/zulip,verma-varsha/zulip,eeshangarg/zulip,shubhamdhama/zulip,Galexrt/zulip,showell/zulip,dhcrzf/zulip,dhcrzf/zulip,dhcrzf/zulip,mahim97/zulip,showell/zulip,rht/zulip,mahim97/zulip,synicalsyntax/zulip,rht/zulip,timabbott/zulip,synicalsyntax/zulip,vaidap/zulip,eeshangarg/zulip,vaidap/zulip,eeshangarg/zulip,vabs22/zulip,shubhamdhama/zulip,verma-varsha/zulip,verma-varsha/zulip,eeshangarg/zulip,brainwane/zulip,dhcrzf/zulip,dhcrzf/zulip,brainwane/zulip,tommyip/zulip,andersk/zulip,rht/zulip,rishig/zulip,timabbott/zulip,punchagan/zulip,timabbott/zulip,amanharitsh123/zulip,kou/zulip,synicalsyntax/zulip,hackerkid/zulip,mahim97/zulip,jrowan/zulip,punchagan/zulip,kou/zulip,kou/zulip,dhcrzf/zulip,tommyip/zulip,Galexrt/zulip,hackerkid/zulip,vaidap/zulip,shubhamdhama/zulip,hackerkid/zulip,rht/zulip,amanharitsh123/zulip,brockwhittaker/zulip,rishig/zulip,jackrzhang/zulip,hackerkid/zulip,Galexrt/zulip,hackerkid/zulip,rht/zulip,punchagan/zulip,Galexrt/zulip,showell/zulip,synicalsyntax/zulip,vaidap/zulip,tommyip/zulip,jrowan/zulip,timabbott/zulip,mahim97/zulip,brockwhittaker/zulip,vaidap/zulip,kou/zulip,kou/zulip,kou/zulip,zulip/zulip,mahim97/zulip,zulip/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,andersk/zulip,brockwhittaker/zulip,shubhamdhama/zulip,rishig/zulip,verma-varsha/zulip,jackrzhang/zulip,vabs22/zulip,andersk/zulip,zulip/zulip,vabs22/zulip,jrowan/zulip,showell/zulip,Galexrt/zulip,timabbott/zulip,amanharitsh123/zulip,brainwane/zulip,shubhamdhama/zulip,rht/zulip,mahim97/zulip,amanharitsh123/zulip,verma-varsha/zulip,Galexrt/zulip,brainwane/zulip,jackrzhang/zulip,kou/zulip,amanharitsh123/zulip,showell/zulip,synicalsyntax/zulip,verma-varsha/zulip,rishig/zulip,punchagan/zulip,vabs22/zulip,brockwhittaker/zulip,jrowan/zulip,brockwhittaker/zulip,brockwhittaker/zulip,jackrzhang/zulip,vabs22/zulip,tommyip/zulip,zulip/zulip,shubhamdhama/zulip,vabs22/zulip,tommyip/zulip,eeshangarg/zulip