Change deploy strategy for TravisCI

怒られたしよく失敗するので変更してみる

```
The default strategy for Heroku deployments is currently "anvil".
This will be changed to "api" in the near future.
Consider setting it explicitly to "api" or "git".

```
Merge pull request #14 from moqada/feature/fix-travis-settings

Change deploy strategy for TravisCI