Revert "Specify articleLayout.container to flex in column direction (reactjs#1414)" (reactjs#1415)

This reverts commit d0718a3.

Author: gaearon

src/theme.js

@@ -115,7 +115,6 @@ const sharedStyles = {
   articleLayout: {
     container: {
       display: 'flex',
-      flexDirection: 'column',
       minHeight: 'calc(100vh - 60px)',
       [media.greaterThan('sidebarFixed')]: {
         maxWidth: 840,