TravisCI: do not update rubygems
TravisCI: do not update rubygems

```
$ gem update --system
Updating rubygems-update
ERROR:  Error installing rubygems-update:
	rubygems-update requires Ruby version >= 2.3.0.
ERROR:  While executing gem ... (NoMethodError)
    undefined method `version' for nil:NilClass
The command "gem update --system" failed and exited with 1 during .
```

kmuto/review,kmuto/review,takahashim/review,kmuto/review,takahashim/review