Update .travis.yml; don't test Node.js 6 & 8

Author: fisker

.travis.yml

@@ -1,14 +1,14 @@
 language: node_js
 node_js:
-  - 12
-  - 10
-  - 8
-  - 6
+  - '13'
+  - '12'
+  - '10'
 matrix:
   include:
-    - node_js: 12
+    - node_js: '12'
       env: INTEGRATION=true
 script:
-  - if [ $INTEGRATION == true ]; then npm run integration; fi
+  - if [[ $INTEGRATION == true ]]; then npm run integration; fi
 before_install: npm install --global npm
-after_success: npx codecov --file=./coverage/lcov.info
+after_success:
+  - if [[ $INTEGRATION != true ]]; then npx codecov --file=./coverage/lcov.info; fi