chore: use yarn as task runner only

Completed switch from npm to yarn. Closes #82.

.travis.yml

@@ -1,8 +1,8 @@
 language: node_js
 script:
-  - npm run format:check
-  - npm run lint
-  - npm run test:ci
-  - npm run test:typings
-  - npm run coveralls
-  - npm run build
+  - yarn run format:check
+  - yarn run lint
+  - yarn run test:ci
+  - yarn run test:typings
+  - yarn run coveralls
+  - yarn run build

package.json

@@ -12,7 +12,7 @@
     "src/"
   ],
   "scripts": {
-    "build": "npm run build:browser && npm run build:commonjs && npm run build:es",
+    "build": "yarn run build:browser && yarn run build:commonjs && yarn run build:es",
     "build:browser": "webpack --config ./config/webpack.umd.js",
     "build:clean": "rimraf dist",
     "build:commonjs": "tsc --project tsconfig.build.json  --module CommonJS --outDir dist/cjs",
@@ -28,7 +28,7 @@
     "format:check": "prettier --list-different \"src/**/*.ts\"",
     "format:data": "prettier --write \"data/**/*.json\"",
     "lint": "tslint --project tsconfig.json \"src/**/*.ts\"",
-    "prepublishOnly": "npm run build",
+    "prepublishOnly": "yarn run build",
     "test": "jest --watch",
     "test:check": "jest --ci",
     "test:ci": "jest --ci --coverage --runInBand",