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
- [ ] 1.9.3
- [ ] 1.9.2
- [ ] 1.8.7

- Resolves #1088
Yeah, I thought so too. But I was seeing it in the output. However, I just setup a sample script and ran it manually; no warnings:

```ruby
# warn_test.rb
_ = "not used"
_test = "not used"

p @ivar
puts "script ran"
```

Output is as expected:

```console
$ ruby -w -W2 warn_test.rb
var_warning.rb:4: warning: instance variable @notset not initialized
nil
am I working
```

I tried with Ruby 2.1.2, 2.0.0, 1.9.3, and 1.8.7.

I also don't see it in the travis logs. I'll look into my environment to see what's up. :(