Lessen strictness of zlib tests #773

@flippmoke - looks like the nodejs core team decided NOT to rollback the zlib upgrade (nodejs/node#12404) so the problem at nodejs/node#12244 still holds.

I propose therefore adapting the node-mapnik tests to:

• Stop comparing results between mapnik and node (or flip the one bit so that the tests pass)
• Per earlier discussion, use the higher level zlib.inflate/deflate API (like this) rather than zlib.createGzip
• Move .travis.yml and appveyor.yml to using latest version of v4, v6, v7, and v8 (rather than pinned versions)