The following rules have been clarified for this campaign and are based on feedback from the Paizo Forums and Jason Bulmahn.
Q. Can an Animal companion get any other feats (without increasing its intelligence) other than those listed in the Core Rule Book?
A. Not without GM approval. [GM EDIT: Animal Companions will be limited to those feats listed under animal companions in the Core Rule Book]
Q. What happens to an Animal Companion which it gets an Intelligence of 3?
A. Animal companions will not be allowed to have their Intelligence Score raised beyond 2. Should an animal companion’s Intelligence be raised to more than 2, it ceases to be an animal companion.
Q. Can I improve my companion’s Intelligence to 3 or higher and give it weapon feats?
A. No. An Intelligence of 3 does not grant animals sentience, the ability to use weapons or tools, speak a language (though they may understand one with a rank in Linguistics; this does not grant literacy), or activate magic devices. Also note that raising an animal companion’s Intelligence to 3 or higher does not eliminate the need to make Handle Animal checks to direct its actions; even semi-intelligent animals still act like animals unless trained not to. Please note that in any case, as per this campaign, and animal companion can not have an intelligence score higher than 2.