Test for activemodel instead of rails for activemodel validations

There's a lot of use-cases where ActiveModel gets used without
including the full rails stack. Since nothing in activemodel
validations depends on rails, we can just test if activemodel is
available before including it, instead of testing for rails.

Thanks to @Xylakant

Reference: https://github.com/couchbase/couchbase-ruby-model/pull/12
Change-Id: I25c311e49836cdf21e126510198b4fd0e9d0e170
Reviewed-on: http://review.couchbase.org/26137
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>