Add Node.js 12 to CI Services (#333)

diff --git a/.travis.yml b/.travis.yml
index 7e44fef..dd562ab 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,9 +5,10 @@
   depth: 10
 
 node_js:
-  - "6"
-  - "8"
-  - "10"
+  - 6
+  - 8
+  - 10
+  - 12
 
 install:
   - npm install