fix(forms): Changes input text color to be darker.

[Finishes #103033628]

BREAKING CHANGE: (sass variable) $input-color is now $neutral-3.

src/pivotal-ui/components/pui-variables.scss

@@ -501,7 +501,7 @@ $input-bg-disabled:              $gray-lighter !default;
 $progress-border-radius:         $border-radius-base !default;
 $input-font-weight:              $font-weight-em-high;
 
-$input-color:                    $neutral-4 !default;
+$input-color:                    $neutral-3 !default;
 $input-color-hover:              $accent-5;
 $input-color-active:             $accent-3;
 $input-border:                   $neutral-7 !default;