Migrate to yarn and update dependencies (#58)

Removed `package-lock.json` and added `yarn.lock` instead.

Even if there are outdated version in package.json, yarn installed up-to-date versions. The only problem is with flow@0.78, it returns me an error which I can not figure out:

<details>
  <summary>Flow@0.78 error</summary>
  
```
$ yarn flow
yarn run v1.6.0
$ flow
Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ /private/tmp/flow/flowlib_316436e7/webassembly.js:11:52

Function [1] is not a polymorphic type.

     /private/tmp/flow/flowlib_316436e7/webassembly.js
        8│ type ResultObject = { module: WebAssembly$Module, instance: WebAssembly$Instance };
        9│
       10│ // https://github.com/WebAssembly/design/blob/master/JS.md#exported-function-exotic-objects
       11│ declare class ExportedFunctionExoticObject extends Function {
         │ ---------------------------------------------------^^^^^^^^--------------------------------
       12│   (): mixed;
       13│ }
       14│

     scripts/babel-nodes.js
 [1] 1910│ type Function = (
         │ -----^^^^^^^^------------------------------------------------------------------------------
```
</details>

Author: ezhlobo

.gitignore

@@ -1,5 +1,7 @@
 .DS_Store
 npm-debug.log
+package-lock.json
+yarn-error.log
 node_modules
 /coverage
 /lib

.travis.yml

@@ -2,7 +2,7 @@ language: node_js
 node_js:
   - "6"
 script:
-  - npm run build
-  - npm test
-  - npm run flow
-  - npm run lint
+  - yarn build
+  - yarn test
+  - yarn flow
+  - yarn lint