Update popper to 1.12.3

Author: sesemaya

_config.yml

@@ -45,8 +45,8 @@
     jquery_hash:    "sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN"
     md_font:        "https://fonts.googleapis.com/css?family=Roboto:300,300i,400,400i,500,500i,700,700i"
     md_icon:        "https://fonts.googleapis.com/icon?family=Material+Icons"
-    popper:         "https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.11.0/umd/popper.min.js"
-    popper_hash:    "sha384-b/U6ypiBEHpOf/4+1nzFpr53nxSS+GLCkfwBdFNTxtclqqenISfwAzpKaMNFNmj4"
+    popper:         "https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.3/umd/popper.min.js"
+    popper_hash:    "sha384-vFJXuSJphROIrBnz7yo7oB41mKfc8JzQZiCq4NCceLEaO4IHwicKwpJf9c9IpFgh"
   current_version:  "4.0.0-beta"
   docs_version:     "4.0"
   download: