Run "gem update" to avoid Gem::Ext::BuildError
Run "gem update" to avoid Gem::Ext::BuildError

The following error occurs when installing rainbow gem:

```
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    can't modify frozen String
```
ref. https://github.com/sickill/rainbow/issues/49

yasaichi/gakubuchi,yasaichi/gakubuchi,yasaichi/gakubuchi