Remove node 8 from travis since 10 is LTS (#1377)

Even though we officially support node 8 as well (package.json stays the same), we only test the latest LTS on Travis.

DEV

.travis.yml

@@ -1,7 +1,5 @@
 language: node_js
-node_js:
-  - '8'
-  - '10'
+node_js: '10'
 script:
   - if [ "$TO_TEST" = "regular" ]; then yarn test-benchmark && yarn test-travis; fi
   - if [ "$TO_TEST" = "integration" ]; then yarn test-integration; fi
@@ -18,7 +16,3 @@ notifications:
     on_success: never
     on_failure: always
     on_pull_requests: false
-matrix:
-  exclude:
-  - node_js: '8'
-    env: TO_TEST=integration