readme: tell people to use alpha-r2748 instead of release-1.5.27.1-beta
readme: tell people to use alpha-r2748 instead of release-1.5.27.1-beta

pagespeed/ngx_pagespeed,digideskio/ngx_pagespeed,digideskio/ngx_pagespeed,0x0mar/ngx_pagespeed,wanrui/ngx_pagespeed,pagespeed/ngx_pagespeed,0x0mar/ngx_pagespeed,wanrui/ngx_pagespeed