time/ruby-iso8601: update to 0.11.0

Due to sysutils/ruby-chef's dependency, stop updating to latest version
0.12.0 for now.

## 0.11.0

* Add support for unary minus operator (thanks @walterbrebels).

## 0.10.1

* Fix `TimeInterval` size for non UTC timezones. (#41)

## 0.10.0

* Fix `TimeInterval` with `<duration>/<end>` patterns and leap years.
* Fix decimal fractions on date atoms. **WARNING** some duration patterns are
  no longer valid.

(taca)