Fix before and after selectors

sass/elements/button.sass

@@ -138,7 +138,7 @@ $button-static-border-color: $grey-lighter !default
           box-shadow: none
           color: $color
       &.is-loading
-        &:after
+        &::after
           border-color: transparent transparent $color-invert $color-invert !important
       &.is-outlined
         background-color: transparent
@@ -150,7 +150,7 @@ $button-static-border-color: $grey-lighter !default
           border-color: $color
           color: $color-invert
         &.is-loading
-          &:after
+          &::after
             border-color: transparent transparent $color $color !important
         &[disabled]
           background-color: transparent
@@ -189,7 +189,7 @@ $button-static-border-color: $grey-lighter !default
   &.is-loading
     color: transparent !important
     pointer-events: none
-    &:after
+    &::after
       +loader
       +center(1em)
       position: absolute !important