Render tests crash under node 9

[jfirebaugh]

Platform: node 9.8.0
Mapbox SDK version: release-boba as of 3294f27

The node render tests crash with the following error message when run under node 9.8.0:

�[32m* passed regressions mapbox-gl-js#4860�[39m
/usr/local/bin/node[34386]: ../src/node_zlib.cc:189:static void node::(anonymous namespace)::ZCtx::Write(const FunctionCallbackInfo<v8::Value> &) [async = false]: Assertion `Buffer::HasInstance(args[4])' failed.
 1: node::Abort() [/usr/local/bin/node]
 2: node::Assert(char const* const (*) [4]) [/usr/local/bin/node]
 3: void node::(anonymous namespace)::ZCtx::Write<false>(v8::FunctionCallbackInfo<v8::Value> const&) [/usr/local/bin/node]
 4: v8::internal::FunctionCallbackArguments::Call(void (*)(v8::FunctionCallbackInfo<v8::Value> const&)) [/usr/local/bin/node]
 5: v8::internal::MaybeHandle<v8::internal::Object> v8::internal::(anonymous namespace)::HandleApiCallHelper<false>(v8::internal::Isolate*, v8::internal::Handle<v8::internal::HeapObject>, v8::internal::Handle<v8::internal::HeapObject>, v8::internal::Handle<v8::internal::FunctionTemplateInfo>, v8::internal::Handle<v8::internal::Object>, v8::internal::BuiltinArguments) [/usr/local/bin/node]
 6: v8::internal::Builtin_Impl_HandleApiCall(v8::internal::BuiltinArguments, v8::internal::Isolate*) [/usr/local/bin/node]
 7: 0xd11dff842fd

Node 8 works correctly.

[mikemorris]

Possibly related to the underlying cause of pngjs/pngjs#95

[jfirebaugh]

We're not planning to support odd-numbered node releases.