Fixed build for Node >=12

[paulrupek]

This should fix #128 and #129. See my comment in the former issue for details.

[philipptrenz]

Would like to test, but have no write access 😕

[lwille]

Hi @pawelrupek,

please adjust the .travis.yml like this to see if your changes compile well against node 12:

diff --git a/.travis.yml b/.travis.yml
index ba2642c..a1ada0d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,6 +5,7 @@ script:
 - npm run cpplint
 language: node_js
 node_js:
+- lts/erbium
 - lts/dubnium
 - '0.12'
 - '0.10'

update: tests look great, going forward with the merge after having adjusted the versions we test against.