Add support for min/max system verions

@colszowka I opened a PR for this to see if you had any input. The major change is that this gem would now use a newer (higher version) system binary by default.

For example, if version 1.9.7.1 of this gem was installed and the system already had a phantomjs binary of version 1.9.8, it would use the system binary.

It's also possible to customize the acceptable binary range:

```ruby
Phantomjs.minimum = '1.8'
Phantomjs.maximum = '1.999'
```

If you don't have any concerns, I'll merge this branch in a few days.

Also, this will resolve PR #25 .