Update all Macs to 10.13

[jdm]

Now that mac4 and mac7 are on 10.13 (since we had to upgrade them by hand rather than macstadium doing it), it probably makes sense to bring all of the other machines up to the same version.

• servo-mac1
• servo-mac2
• servo-mac3
• servo-mac5
• servo-mac6
• servo-mac8
• servo-mac9

[SimonSapin]

It seems that 4 and 7 have been rolled back:

$ for i in $(seq 9); (echo -n "servo-mac$i "; ssh servo-mac$i.servo.org sw_vers|grep ProductVersion)
servo-mac1	ProductVersion:	10.11.6
servo-mac2	ProductVersion:	10.11.6
servo-mac3	ProductVersion:	10.11.6
servo-mac4	ProductVersion:	10.11.6
servo-mac5	ProductVersion:	10.11.6
servo-mac6	ProductVersion:	10.11.6
servo-mac7	ProductVersion:	10.11.6
servo-mac8	ProductVersion:	10.11.6
servo-mac9	ProductVersion:	10.11.6