chore: remove Node 4 from the build matrix, add Node 10

Author: bhovhannes

.travis.yml

@@ -6,7 +6,7 @@ notifications:
     on_success: change
 node_js:
   - '6'
-  - '4'
+  - '8'
   - '10'
 
 env:
@@ -18,7 +18,7 @@ env:
 
 matrix:
   include:
-    - node_js: '6'
+    - node_js: '8'
       env: CI_MODE=saucelabs
       addons:
         sauce_connect: true