Update big.js.d.ts

otherwise typescript will make a script that will try to access a variable that doesn't exist

https://github.com/Microsoft/TypeScript/wiki/What's-new-in-TypeScript#const-enum-completely-inlined-enums

big.js/big.js.d.ts

@@ -6,7 +6,7 @@
 
 declare module BigJsLibrary {
 
-    export enum RoundingMode {
+    export const enum RoundingMode {
         RoundTowardsZero = 0,
         RoundTowardsNearestAwayFromZero = 1,
         RoundTowardsNearestTowardsEven = 2,
@@ -200,4 +200,4 @@ declare module BigJsLibrary {
     }
 }
 
-declare var Big: BigJsLibrary.BigJS;
+declare var Big: BigJsLibrary.BigJS;