Fixed outdated CSS syntax in linear-gradient (bootstrap-datepicker)

Outdated syntax causes warnings with postcss.

- postcss/autoprefixer#530
- https://developer.mozilla.org/en/docs/Web/CSS/linear-gradient

PS. I know this addresses an issue from a 3rd party component. However, bootstrap-datapicker seems unmaintained. Or if you're using a fork, kindly let me know and I'll be happy to PR to that repo instead

src/inputs/date/bootstrap-datepicker/css/datepicker.css

@@ -105,7 +105,7 @@
   background-image: -webkit-gradient(linear, 0 0, 0 100%, from(#fdd49a), to(#fdf59a));
   background-image: -webkit-linear-gradient(top, #fdd49a, #fdf59a);
   background-image: -o-linear-gradient(top, #fdd49a, #fdf59a);
-  background-image: linear-gradient(top, #fdd49a, #fdf59a);
+  background-image: linear-gradient(to bottom, #fdd49a, #fdf59a);
   background-repeat: repeat-x;
   filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#fdd49a', endColorstr='#fdf59a', GradientType=0);
   border-color: #fdf59a #fdf59a #fbed50;
@@ -170,7 +170,7 @@
   background-image: -webkit-gradient(linear, 0 0, 0 100%, from(#f3c17a), to(#f3e97a));
   background-image: -webkit-linear-gradient(top, #f3c17a, #f3e97a);
   background-image: -o-linear-gradient(top, #f3c17a, #f3e97a);
-  background-image: linear-gradient(top, #f3c17a, #f3e97a);
+  background-image: linear-gradient(to bottom, #f3c17a, #f3e97a);
   background-repeat: repeat-x;
   filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#f3c17a', endColorstr='#f3e97a', GradientType=0);
   border-color: #f3e97a #f3e97a #edde34;
@@ -222,7 +222,7 @@
   background-image: -webkit-gradient(linear, 0 0, 0 100%, from(#b3b3b3), to(#808080));
   background-image: -webkit-linear-gradient(top, #b3b3b3, #808080);
   background-image: -o-linear-gradient(top, #b3b3b3, #808080);
-  background-image: linear-gradient(top, #b3b3b3, #808080);
+  background-image: linear-gradient(to bottom, #b3b3b3, #808080);
   background-repeat: repeat-x;
   filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#b3b3b3', endColorstr='#808080', GradientType=0);
   border-color: #808080 #808080 #595959;
@@ -273,7 +273,7 @@
   background-image: -webkit-gradient(linear, 0 0, 0 100%, from(#0088cc), to(#0044cc));
   background-image: -webkit-linear-gradient(top, #0088cc, #0044cc);
   background-image: -o-linear-gradient(top, #0088cc, #0044cc);
-  background-image: linear-gradient(top, #0088cc, #0044cc);
+  background-image: linear-gradient(to bottom, #0088cc, #0044cc);
   background-repeat: repeat-x;
   filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#0088cc', endColorstr='#0044cc', GradientType=0);
   border-color: #0044cc #0044cc #002a80;
@@ -345,7 +345,7 @@
   background-image: -webkit-gradient(linear, 0 0, 0 100%, from(#0088cc), to(#0044cc));
   background-image: -webkit-linear-gradient(top, #0088cc, #0044cc);
   background-image: -o-linear-gradient(top, #0088cc, #0044cc);
-  background-image: linear-gradient(top, #0088cc, #0044cc);
+  background-image: linear-gradient(to bottom, #0088cc, #0044cc);
   background-repeat: repeat-x;
   filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#0088cc', endColorstr='#0044cc', GradientType=0);
   border-color: #0044cc #0044cc #002a80;