Bump yarn, bump dev

.travis.yml

@@ -1,6 +1,9 @@
 language: node_js
 node_js:
   - "10"
+before_install:
+  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.10.1
+  - export PATH=$HOME/.yarn/bin:$PATH
 cache:
   yarn: true
   directories:

package.json

@@ -18,6 +18,7 @@
     "@types/node": "^10.12.0"
   },
   "devDependencies": {
-    "@polkadot/dev": "^0.21.1"
+    "@babel/core": "^7.1.2",
+    "@polkadot/dev": "^0.21.2"
   }
 }