Fix Ruby warnings

Start of fixing warnings provided by Ruby.

Still a **WIP** not ready to merge yet.

Ruby versions:

- [x] 2.1.2
- [ ] 2.1.1
- [ ] 2.1.0
- [ ] 2.0.0
- [x] 1.9.3
- [ ] 1.9.2
- [ ] 1.8.7

- Resolves #1088
Figured it out. It happens _only_ during a `require` call. I can now reproduce with just ruby:

```ruby
# var_warnings.rb
require "./other"

puts "script ran"

###################
# other.rb
_ = "should be ignored"
_test = "not used"

puts "in other"
```

When run with `ruby -w`:

```console
$ ruby -w var_warning.rb
other.rb:2: warning: assigned but unused variable - _test
in other
script ran
```