[Travis] Switch to Xcode 6.4
[Travis] Switch to Xcode 6.4

See https://twitter.com/travisci/status/616154472897773568.
preetipantnagar1989/ObjectMapper,ikesyo/ObjectMapper,zhaoguohui/ObjectMapper,stephanecopin/ObjectMapper,msfeldstein/ObjectMapper,adachic/ObjectMapper,adachic/ObjectMapper,zhaoguohui/ObjectMapper,msdgwzhy6/ObjectMapper,msfeldstein/ObjectMapper,ken0nek/ObjectMapper,Hearst-DD/ObjectMapper,hsavit1/ObjectMapper,hsavit1/ObjectMapper,msdgwzhy6/ObjectMapper,csjlengxiang/ObjectMapper,goodwall/ObjectMapper,teamcarma/IOS-ObjectMapperDeep,nbarnold01/ObjectMapper,nbarnold01/ObjectMapper,stephanecopin/ObjectMapper,tuanphung/ObjectMapper,baydet/ObjectMapper,kota/ObjectMapper,gunosy/ObjectMapper,fullstackclub/ObjectMapper,RyanCodes/ObjectMapper,teamcarma/IOS-ObjectMapperDeep,tuanphung/ObjectMapper,gunosy/ObjectMapper,RyanCodes/ObjectMapper,kota/ObjectMapper,Hearst-DD/ObjectMapper,preetipantnagar1989/ObjectMapper,yoshinorisano/ObjectMapper,rodrigo-lima/ObjectMapper,BlackcombSoftware/ObjectMapper,stephanecopin/ObjectMapper,tonyli508/ObjectMapperDeep,cfraz89/ObjectMapper,kota/ObjectMapper,RyanCodes/ObjectMapper,bscarano/ObjectMapper,bscarano/ObjectMapper,cfraz89/ObjectMapper,baydet/ObjectMapper,ikesyo/ObjectMapper,tonyli508/ObjectMapperDeep,ken0nek/ObjectMapper,rodrigo-lima/ObjectMapper,msfeldstein/ObjectMapper,msdgwzhy6/ObjectMapper,fullstackclub/ObjectMapper,adachic/ObjectMapper,CINEPASS/ObjectMapper,goodwall/ObjectMapper,tuanphung/ObjectMapper,csjlengxiang/ObjectMapper,preetipantnagar1989/ObjectMapper,Busta117/ObjectMapper,Busta117/ObjectMapper,rodrigo-lima/ObjectMapper,fullstackclub/ObjectMapper,BlackcombSoftware/ObjectMapper,nbarnold01/ObjectMapper,yoshinorisano/ObjectMapper,CINEPASS/ObjectMapper,teamcarma/IOS-ObjectMapperDeep,gunosy/ObjectMapper