chore(ci): refactor travis file

- use double quotes on strings
- add slack notifications
- do not cache "node_modules"

.travis.yml

@@ -2,19 +2,20 @@ language:
   - node_js
 
 node_js:
-  - 6
-  - 8
-  - 10
-
-branches:
-  except:
-    - /^v\d+\.\d+\.\d+$/
+  - "10"
+  - "8"
+  - "6"
 
 cache:
   directories:
-    - $HOME/.npm
-    - node_modules
-    - .eslintcache
+    - "$HOME/.npm"
+    - ".eslintcache"
+
+if: tag IS blank # do not build tags
+
+notifications:
+  slack:
+    secure: "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"
 
 before_install:
   - dpkg --compare-versions `npm -v` ge 6.0 || npm i -g npm@latest
@@ -30,7 +31,7 @@ script:
   - npm run cover
 
 after_success:
-  - test $TRAVIS_NODE_VERSION = 8 && bash <(curl -s https://codecov.io/bash)
+  - bash <(curl -s https://codecov.io/bash)
 
 after_script:
  - greenkeeper-lockfile-upload