Fixed OS X build issue in User-Agent header where API is not available.
Fixed OS X build issue in User-Agent header where API is not available.

gauravds/Alamofire,agustindc-rga/Alamofire,meteochu/Alamofire,tonyli508/AlamofireDomain,Noobish1/Alamofire,richardPFisk/Alamofire,iosheadhunter/Alamofire,mbhuta/Alamofire,Vkt0r/Alamofire,Farteen/Alamofire,Noobish1/Alamofire,Koalappp/Alamofire,yimajo/Alamofire,easyui/Alamofire,agustindc-rga/Alamofire,Holmusk/Alamofire,Alamofire/Alamofire,richardPFisk/Alamofire,Vkt0r/Alamofire,Holmusk/Alamofire,antigp/Alamofire,twobitlabs/Alamofire,meteochu/Alamofire,kiancheong/Alamofire,yimajo/Alamofire,jschmid/Alamofire,mbhuta/Alamofire,DouKing/Alamofire,DouKing/Alamofire,Lomotif/Alamofire,iosheadhunter/Alamofire,antigp/Alamofire,dgdosen/Alamofire,Lomotif/Alamofire,Farteen/Alamofire,Koalappp/Alamofire,jschmid/Alamofire,tonyli508/AlamofireDomain,kiancheong/Alamofire,Holmusk/Alamofire,gauravds/Alamofire,dgdosen/Alamofire,Alamofire/Alamofire,twobitlabs/Alamofire,easyui/Alamofire