chore: switch from istanbul to nyc

because istanbul is deprecated

.gitignore

@@ -55,3 +55,4 @@ npm-debug.log
 .directory
 ._*
 *.iml
+.nyc_output

.travis.yml

@@ -1,12 +1,23 @@
 sudo: false
+cache:
+  directories:
+    - node_modules
 language: "node_js"
 node_js:
   - "node"
   - "lts/*"
+matrix: 
+  include:
+    - node_js: "lts/*"
+      env: STATIC_CHECKS=true
 script:
-  - npm install
-  - npm run coverage
-  - npm run lint
+  - |
+    if [ "${STATIC_CHECKS}" = "true" ]; then
+      # Just perform static checks and exit
+      eslint --max-warnings 0 .
+      exit 0
+    fi
+  - nyc mocha
 after_script:
   - npm install coveralls
-  - cat ./coverage/lcov.info | coveralls
+  - nyc report --reporter=text-lcov | coveralls