Bump ZF2 version requirement

zendframework/ZendDeveloperTools#161 proposes to revert zendframework/ZendDeveloperTools#158, which was introduced due to a zendframework/ZendDeveloperTools#157, which was introduced due to a a ZDT listener returning a boolean `true`, which would cause short-circuiting of authorization listeners (and, consequently, cause any user to be considered authorized!). Since ZF 2.3.2 fixes the root cause, by pinning to that, we can be assured that when zendframework/ZendDeveloperTools#161 is merged, ZDT and Apigility will continue to play well together.
Merge branch 'hotfix/zf-version'

Close #76