update button borders to match bgs for easier outline button styling

scss/_variables.scss

@@ -151,7 +151,7 @@ $btn-font-weight:                normal !default;
 
 $btn-primary-color:              #fff !default;
 $btn-primary-bg:                 $brand-primary !default;
-$btn-primary-border:             darken($btn-primary-bg, 5%) !default;
+$btn-primary-border:             $btn-primary-bg !default;
 
 $btn-secondary-color:            $gray-dark !default;
 $btn-secondary-bg:               #fff !default;
@@ -159,19 +159,19 @@ $btn-secondary-border:           #ccc !default;
 
 $btn-info-color:                 #fff !default;
 $btn-info-bg:                    $brand-info !default;
-$btn-info-border:                darken($btn-info-bg, 5%) !default;
+$btn-info-border:                $btn-info-bg !default;
 
 $btn-success-color:              #fff !default;
 $btn-success-bg:                 $brand-success !default;
-$btn-success-border:             darken($btn-success-bg, 5%) !default;
+$btn-success-border:             $btn-success-bg !default;
 
 $btn-warning-color:              #fff !default;
 $btn-warning-bg:                 $brand-warning !default;
-$btn-warning-border:             darken($btn-warning-bg, 5%) !default;
+$btn-warning-border:             $btn-warning-bg !default;
 
 $btn-danger-color:               #fff !default;
 $btn-danger-bg:                  $brand-danger !default;
-$btn-danger-border:              darken($btn-danger-bg, 5%) !default;
+$btn-danger-border:              $btn-danger-bg !default;
 
 $btn-link-disabled-color:        $gray-light !default;