Add yarn

.gitignore

@@ -13,5 +13,4 @@
 
 node_modules
 dist
-yarn.lock
-package-lock.json
+package-lock.json

.travis.yml

@@ -21,5 +21,7 @@ before_install:
       bigchaindb/bigchaindb:0.10.2
       start
 
+script: yarn test
+
 notifications:
     email: false

package.json

@@ -23,7 +23,7 @@
     "release": "./node_modules/release-it/bin/release.js --src.tagName='v%s' --github.release --npm.publish --non-interactive",
     "release-minor": "./node_modules/release-it/bin/release.js minor --src.tagName='v%s' --github.release --npm.publish --non-interactive",
     "release-major": "./node_modules/release-it/bin/release.js major --src.tagName='v%s' --github.release --npm.publish --non-interactive",
-    "prepublish": "npm update && npm run build",
+    "prepublishOnly": "npm update && npm run build",
     "precommit": "npm run lint"
   },
   "devDependencies": {