Update changelog for 2.7.4.
Update changelog for 2.7.4.

Conflicts:
	CHANGELOG.md

artem-zinnatullin/okhttp,square/okhttp,wfxiang08/okhttp,cketti/okhttp,SunnyDayDev/okhttp,Voxer/okhttp,ze-pequeno/okhttp,ansman/okhttp,mjbenedict/okhttp,NightlyNexus/okhttp,cnoldtree/okhttp,NightlyNexus/okhttp,cketti/okhttp,ansman/okhttp,yschimke/okhttp,Synix/okhttp,yakatak/okhttp,rschilling/okhttp,tiarebalbi/okhttp,mjbenedict/okhttp,nfuller/okhttp,square/okhttp,cnoldtree/okhttp,nfuller/okhttp,hgl888/okhttp,germanattanasio/okhttp,artem-zinnatullin/okhttp,mjbenedict/okhttp,tiarebalbi/okhttp,SunnyDayDev/okhttp,germanattanasio/okhttp,SunnyDayDev/okhttp,qingsong-xu/okhttp,wfxiang08/okhttp,yakatak/okhttp,rschilling/okhttp,jrodbx/okhttp,jrodbx/okhttp,hgl888/okhttp,tiarebalbi/okhttp,square/okhttp,yschimke/okhttp,zmarkan/okhttp,Synix/okhttp,NightlyNexus/okhttp,germanattanasio/okhttp,rschilling/okhttp,Voxer/okhttp,Voxer/okhttp,yschimke/okhttp,yakatak/okhttp,cmzy/okhttp,wfxiang08/okhttp,nfuller/okhttp,qingsong-xu/okhttp,zmarkan/okhttp,ze-pequeno/okhttp,cnoldtree/okhttp,zmarkan/okhttp,Synix/okhttp,hgl888/okhttp,ze-pequeno/okhttp,cketti/okhttp,cmzy/okhttp,qingsong-xu/okhttp,ansman/okhttp,artem-zinnatullin/okhttp,jrodbx/okhttp