Remove rubygems require

Remove unnecessary rubygems require

Fix ruby warnings as well

```bash
lib/roodi/checks/abc_metric_method_check.rb:40: warning: shadowing outer local variable - node
lib/roodi/checks/abc_metric_method_check.rb:47: warning: shadowing outer local variable - node
lib/roodi/checks/abc_metric_method_check.rb:54: warning: shadowing outer local variable - node
lib/roodi/checks/check.rb:31: warning: shadowing outer local variable - node
lib/roodi/checks/check.rb:32: warning: shadowing outer local variable - node
lib/roodi/core/runner.rb:63: warning: assigned but unused variable - e
```

Except this one

```bash
lib/roodi/checks/method_name_check.rb:10: warning: character class has duplicated range: /^[_a-z<>=\[|+-\/\*`]+[_a-z0-9_<>=~@\[\]]*[=!\?]?$/
```