Don't make prepareStackTrace unwritable

This breaks other code (such as Grim) that may want to temporarily
use a different prepareStackTrace function.

src/coffee-script.js

@@ -20,7 +20,13 @@ exports.getCachePath = function(sourceCode) {
 
 exports.compile = function(sourceCode, filePath) {
   if (!CoffeeScript) {
+    var previousPrepareStackTrace = Error.prepareStackTrace
     CoffeeScript = require('coffee-script')
+
+    // When it loads, coffee-script reassigns Error.prepareStackTrace. We have
+    // already reassigned it via the 'source-map-support' module, so we need
+    // to set it back.
+    Error.prepareStackTrace = previousPrepareStackTrace
   }
 
   var output = CoffeeScript.compile(sourceCode, {

src/compile-cache.js

@@ -138,15 +138,6 @@ require('source-map-support').install({
   }
 })
 
-// This prevents coffee-script from reassigning `Error.prepareStackTrace` when
-// it is loaded.
-Object.defineProperty(Error, 'prepareStackTrace', {
-  value: Error.prepareStackTrace,
-  configurable: false,
-  enumerable: true,
-  writable: false
-})
-
 Object.keys(COMPILERS).forEach(function (extension) {
   var compiler = COMPILERS[extension]