Chore: switch yarn to npm (#21)

* Chore: ignore yarn.lock add package-lock.json

* Chore: switch yarn to npm

* Chore: switch yarn to npm

Author: aichbauer

.gitignore

@@ -8,7 +8,7 @@ coverage
 *.log
 *.cache
 .eslintcache
-package-lock.json
+yarn.lock
 
 # macOS
 .DS_STORE

.travis.yml

@@ -5,7 +5,7 @@ node_js:
   - 6
   - 8
 install:
-  - yarn
+  - npm i
   - npm i -g codecov
 script:
   - npm test