Prevent logs in commandExistsWindowsSync. fix #18

The default value of `stdio.options` in `execSync` is 'pipe'. That is *supposed* to be equivalent to `['pipe', 'pipe', 'pipe']`. 
In reality there seems to be a difference here - using the default (or 'pipe') results in the extra log. Using an array solves the issue.

Author: IliaSky

lib/command-exists.js

@@ -89,7 +89,7 @@ var commandExistsUnixSync = function(commandName, cleanedCommandName) {
 
 var commandExistsWindowsSync = function(commandName, cleanedCommandName, callback) {
   try {
-      var stdout = execSync('where ' + cleanedCommandName);
+      var stdout = execSync('where ' + cleanedCommandName, {stdio: []});
       return !!stdout;
   } catch (error) {
       return false;