Don't allow extra characters surrounding valid postal codes

  * "123456" was previously considered a valid US zip code, as it
    contains the valid "12345" zip code
  * Also update the Ecuador regex; otherwise, some of the samples being
    generated were not recognized as valid
We require anyone using 1.8.7 to have [oniguruma](http://www.geocities.jp/kosako3/oniguruma/) installed, which supports the second argument for `#match`. Let me suggest adding this private method to the class, then calling it in the constructor:

```ruby
def build_regexp(regexp_str, modifiers = '')
  if RUBY_VERSION <= "1.8.7"
    begin
      Oniguruma::ORegexp.new(regexp_str, modifiers)
    rescue NameError
      raise "Postal codes require the Oniguruma gem when using Ruby 1.8. Please install, require, and retry."
    end
  else
    Regexp.new(regexp_str, modifiers)
  end
end
```