fix(back-button): fix position

Author: manucorporat

core/src/components/back-button/back-button.ios.scss

@@ -6,7 +6,7 @@
 
 .back-button-ios {
   @include padding(0);
-  @include margin(2px, 0, 0, 0);
+  @include margin(0, 0, 0, 0);
 
   z-index: $back-button-ios-button-z-index;
   overflow: visible;

core/src/components/back-button/back-button.md.scss

@@ -5,7 +5,7 @@
 // --------------------------------------------------
 
 .back-button-md {
-  @include margin(2px, 6px, 0, 0);
+  @include margin(1px, 6px, 0, 0);
   @include padding(0, 5px);
 
   min-width: 44px;