Clarify Twitter Bootstrap dropdown compatibility

Hello FastClick developers,

"Use case 2" in the README describes a compatibility issue with Twitter Bootstrap 2.2.2.  After reading this, I was unsure about whether the issue only applied to version 2.2.2 or whether this workaround was
still required.  After some investigation, it appears that the compatibility issue was introduced in twbs/bootstrap@c9cef741 (included in `v2.1.0` and later) and was resolved in twbs/bootstrap@b5ad5068
(included in `v2.3.0` and later, including `v3.0.0`) as a result of twbs/bootstrap#6488.

To avoid causing confusion for future users who may not know about the state of this issue, this commit clarifies which versions of Twitter Bootstrap are affected.

Thanks for considering,
Kevin