eslint: Update eslint config to have es6 env.
eslint: Update eslint config to have es6 env.

This will allow us to use spread operator.

rht/zulip,tommyip/zulip,jackrzhang/zulip,hackerkid/zulip,punchagan/zulip,rishig/zulip,eeshangarg/zulip,punchagan/zulip,showell/zulip,brainwane/zulip,kou/zulip,dhcrzf/zulip,showell/zulip,tommyip/zulip,hackerkid/zulip,timabbott/zulip,punchagan/zulip,synicalsyntax/zulip,showell/zulip,timabbott/zulip,zulip/zulip,kou/zulip,rht/zulip,brainwane/zulip,shubhamdhama/zulip,tommyip/zulip,zulip/zulip,dhcrzf/zulip,tommyip/zulip,dhcrzf/zulip,eeshangarg/zulip,rishig/zulip,showell/zulip,kou/zulip,timabbott/zulip,andersk/zulip,rishig/zulip,showell/zulip,punchagan/zulip,eeshangarg/zulip,showell/zulip,punchagan/zulip,jackrzhang/zulip,brainwane/zulip,andersk/zulip,brainwane/zulip,rishig/zulip,timabbott/zulip,zulip/zulip,eeshangarg/zulip,hackerkid/zulip,kou/zulip,kou/zulip,rht/zulip,synicalsyntax/zulip,shubhamdhama/zulip,synicalsyntax/zulip,jackrzhang/zulip,hackerkid/zulip,shubhamdhama/zulip,zulip/zulip,brainwane/zulip,synicalsyntax/zulip,rht/zulip,kou/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,andersk/zulip,tommyip/zulip,hackerkid/zulip,andersk/zulip,punchagan/zulip,dhcrzf/zulip,jackrzhang/zulip,andersk/zulip,dhcrzf/zulip,rht/zulip,shubhamdhama/zulip,tommyip/zulip,brainwane/zulip,eeshangarg/zulip,hackerkid/zulip,shubhamdhama/zulip,rht/zulip,brainwane/zulip,rishig/zulip,synicalsyntax/zulip,dhcrzf/zulip,eeshangarg/zulip,timabbott/zulip,hackerkid/zulip,tommyip/zulip,timabbott/zulip,synicalsyntax/zulip,timabbott/zulip,jackrzhang/zulip,andersk/zulip,shubhamdhama/zulip,kou/zulip,showell/zulip,dhcrzf/zulip,synicalsyntax/zulip,jackrzhang/zulip,zulip/zulip,punchagan/zulip,shubhamdhama/zulip,rishig/zulip,rishig/zulip,jackrzhang/zulip