Remove Node 6 and 7 from Travis CI rotation, and add 13.
Remove Node 6 and 7 from Travis CI rotation, and add 13.

The latest version of Meteor (1.9) uses Node 12, and the previous several
versions use Node 8, so Node 6 and 7 are no longer relevant.

Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor