Fix gem installation error on Ruby >= 2.4, ubuntu-16.04, and Azure Pipelines
Fix gem installation error on Ruby >= 2.4, ubuntu-16.04, and Azure Pipelines

```
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /var/lib/gems/2.3.0 directory.
```

emsk/romajify,emsk/romajify