docs/dev-env: Explicitly recommend VirtualBox 5.1.8 exactly.
docs/dev-env: Explicitly recommend VirtualBox 5.1.8 exactly.

showell/zulip,kou/zulip,andersk/zulip,mahim97/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,tommyip/zulip,andersk/zulip,rishig/zulip,mahim97/zulip,dhcrzf/zulip,punchagan/zulip,Galexrt/zulip,zulip/zulip,zulip/zulip,tommyip/zulip,eeshangarg/zulip,dhcrzf/zulip,timabbott/zulip,synicalsyntax/zulip,shubhamdhama/zulip,eeshangarg/zulip,rht/zulip,synicalsyntax/zulip,eeshangarg/zulip,punchagan/zulip,shubhamdhama/zulip,kou/zulip,punchagan/zulip,punchagan/zulip,showell/zulip,hackerkid/zulip,tommyip/zulip,Galexrt/zulip,tommyip/zulip,hackerkid/zulip,jackrzhang/zulip,showell/zulip,hackerkid/zulip,timabbott/zulip,rht/zulip,showell/zulip,rishig/zulip,jackrzhang/zulip,shubhamdhama/zulip,Galexrt/zulip,jackrzhang/zulip,kou/zulip,kou/zulip,rishig/zulip,brainwane/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,rishig/zulip,brockwhittaker/zulip,shubhamdhama/zulip,jackrzhang/zulip,hackerkid/zulip,timabbott/zulip,synicalsyntax/zulip,mahim97/zulip,timabbott/zulip,rht/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,Galexrt/zulip,showell/zulip,brainwane/zulip,Galexrt/zulip,synicalsyntax/zulip,shubhamdhama/zulip,mahim97/zulip,zulip/zulip,jackrzhang/zulip,rht/zulip,synicalsyntax/zulip,punchagan/zulip,zulip/zulip,kou/zulip,brockwhittaker/zulip,brockwhittaker/zulip,hackerkid/zulip,andersk/zulip,brockwhittaker/zulip,brockwhittaker/zulip,timabbott/zulip,synicalsyntax/zulip,zulip/zulip,punchagan/zulip,timabbott/zulip,rishig/zulip,brainwane/zulip,tommyip/zulip,dhcrzf/zulip,showell/zulip,rht/zulip,shubhamdhama/zulip,brainwane/zulip,zulip/zulip,Galexrt/zulip,brainwane/zulip,zulip/zulip,kou/zulip,shubhamdhama/zulip,eeshangarg/zulip,rishig/zulip,dhcrzf/zulip,mahim97/zulip,rht/zulip,dhcrzf/zulip,jackrzhang/zulip,synicalsyntax/zulip,dhcrzf/zulip,eeshangarg/zulip,tommyip/zulip,hackerkid/zulip,Galexrt/zulip,brainwane/zulip,timabbott/zulip,brockwhittaker/zulip,mahim97/zulip,tommyip/zulip,rishig/zulip,jackrzhang/zulip,brainwane/zulip,dhcrzf/zulip,showell/zulip,andersk/zulip