Fix edge case in Server#update when options[:ram] is nil

This pull request fixes a `NoMethodError` in `Server#update` if the `options` hash doesn't contain an element named `:ram`.
```
/usr/lib/ruby/gems/2.0.0/gems/profitbricks-0.9.8/lib/profitbricks/server.rb:31:in `update': undefined method `<' for nil:NilClass (NoMethodError)
```
This case didn't occur before because `options[:ram]` was mandatory (changed in #12).