Fixes issue with tax rounding

- Instead of using a rounding method prone to floating point math issues, this uses a coercion using exp notation

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>

lib/recurly/pricing/calculations.js

@@ -114,8 +114,8 @@ Calculations.prototype.tax = function (done) {
         });
 
         // tax estimation prefers partial cents to always round up
-        self.price.now.tax = Math.ceil(self.price.now.tax * 100) / 100;
-        self.price.next.tax = Math.ceil(self.price.next.tax * 100) / 100;
+        self.price.now.tax = taxCeil(self.price.now.tax);
+        self.price.next.tax = taxCeil(self.price.next.tax);
       }
       done.call(self);
     });
@@ -237,3 +237,16 @@ Calculations.prototype.planPrice = function () {
 function decimal (prop) {
   this[prop] = (Math.round(Math.max(this[prop], 0) * 100) / 100).toFixed(2);
 }
+
+/**
+ * Ceilings the second decimal of a number without risk of
+ * floating point math errors
+ *
+ * @param {Number} number
+ * @return {Number}
+ * @private
+ */
+
+function taxCeil (number) {
+  return +(Math.ceil(number + 'e+2') + 'e-2');
+}