Remove at_exit exception handling monkey patch

This is not an issue with newer versions of ruby 1.9.3 and 2.0.0+ as the exit
code does NOT get reset to 0.

Fixes #846

Example code from: https://bugs.ruby-lang.org/issues/5218

```ruby
at_exit do
  raise "X" rescue nil
end

at_exit do
  nil
end

at_exit do
  exit 1
end

at_exit do
  exit 2
end
```

```
$ ruby -v
ruby 2.0.0p576 (2014-09-19) [x86_64-darwin13.4.0]
$ ruby at_exit.rb; echo $?
1
```

```
$ ruby -v
ruby 1.9.3p547 (2014-05-14) [x86_64-darwin13.4.0]
$ ruby at_exit.rb; echo $?
1
```