Fix common scenarios where xml nodes are not present

Zillow typically returns a rentzestimate in this format:

``` xml
<rentzestimate>
  <amount currency="USD">2505</amount>
  <last-updated>04/01/2013</last-updated>
  <oneWeekChange deprecated="true"></oneWeekChange>
  <valueChange duration="30">50</valueChange>
  <valuationRange>
    <low currency="USD">1979</low>
    <high currency="USD">3056</high>
  </valuationRange>
</rentzestimate>
```

However, sometimes `<valueChange>` doesn't have a `duration` attribute.  

``` xml
<valueChange></valueChange>
```

This was causing Rubillow to throw an error like the following for me:

> NoMethodError: undefined method `value' for nil:NilClass - Zestimateable#extract_zestimate:71

The offending line in Rubillow looked like this:

``` ruby
:value_duration => xml.xpath('//rentzestimate/valueChange').attr('duration').value.chomp
```

The duration attribute is nil, so this raised an error.  Fixed it with this:

``` ruby
:value_duration => xml.xpath('//rentzestimate').first.xpath("//valueChange").first.attr("duration").chomp
```

I added a test to verify that this fixes the issue, and also **bumped the version number to 0.0.6.**  Since this is a patch, I think [SemVer](http://semver.org/) would require a new version number.

**P.S.** Why are we calling [String#chomp](http://ruby-doc.org/core-2.0/String.html#chomp-method) on the end here?  It was already there so I left it in, but it seems unnecessary to me.