Fix the conversion error while converting mm to px (#11)

Example: `auto x = 5.mm` was converted to `0`, now it is fixed to
convert to a float value.

Signed-off-by: Aravinda VK <vkaravinda7@gmail.com>

Author: aravindavk

source/chitra/helpers.d

@@ -17,7 +17,7 @@ double cm(T)(T value)
 
 double mm(T)(T value)
 {
-    return (value / 10).cm;
+    return (value / 10.0).cm;
 }
 
 // 1 pt = 1/72 of inch and 1 px = 1/96 of inch