Deprecate Node 4 and 6 binary publishing

[danpat]

I ran a query over our download logs for the OSRM Node binaries. In the last 30 days:

• nobody has used the 5.20.0 Node 6 build so far
• the 5.18 release, which was stable for a while, saw 20k+ downloads, and only 70 of them were for Node 6 or older.
• There were fewer than 20 Node 4 downloads in total

I'm going to kill off our Node 4 (deprecated) and Node 6 (now in Maintenance mode, which ends April 2019) publishing - no need to wait for Travis to build these if basically nobody is using them.