fix($theme-default): set display block on main tag for IE11

refs vuejs#1154

Author: HerrBertling

packages/@vuepress/theme-default/components/Home.vue

@@ -75,6 +75,7 @@ export default {
   padding $navbarHeight 2rem 0
   max-width 960px
   margin 0px auto
+  display block
   .hero
     text-align center
     img

packages/@vuepress/theme-default/components/Page.vue

@@ -197,6 +197,7 @@ function find (page, items, offset) {
 
 .page
   padding-bottom 2rem
+  display block
 
 .page-edit
   @extend $wrapper