remove inappropriate negative margin on menubar

caused 1px mismatch between header background and menubar
when toolbar is hidden.

IPython/html/static/notebook/less/menubar.less

@@ -5,7 +5,7 b''
   .navbar {
     border-top: 1px;
     border-radius: 0px 0px @border-radius-base @border-radius-base;
-    margin-bottom: -1px;
+    margin-bottom: 0px;
   }
   
   .navbar-toggle {

IPython/html/static/style/style.min.css

@@ -10320,7 +10320,7 b' select[multiple].celltoolbar select {'
 #menubar .navbar {
   border-top: 1px;
   border-radius: 0px 0px 2px 2px;
-  margin-bottom: -1px;
+  margin-bottom: 0px;
 }
 #menubar .navbar-toggle {
   float: left;