fix: re-align popup after width adjustment

[cagen]

fix #27
Force re-align dropdown position after overlay's width is changed.

[coveralls]

Coverage decreased (-1.2%) to 91.25% when pulling 81d5ce2 on cagen:hotfix/fix_align_position into f993dce on react-component:master.

[cagen]

Seems the rc-trigger forcePopupAlign can't work (https://github.com/react-component/trigger/blob/master/src/index.js#L432), where the this._component evals to undefined. I have to use the this.refs.trigger._component.alignInstance.forceAlign() to re-align.

[coveralls]

Coverage increased (+1.3%) to 93.75% when pulling c6309c5 on cagen:hotfix/fix_align_position into f993dce on react-component:master.