Workaround for HHVM not supporting case-insensitive constants

First of all, great job, guys!

I'm automating my tests using Travis CI and ran into this issue when enabling tests under HHVM:

```
Case insensitive constant names are not supported in HipHop
/home/travis/build/goblindegook/Syllables/vendor/10up/wp_mock/WP_Mock/API/constant-mocks.php:39
/home/travis/build/goblindegook/Syllables/vendor/10up/wp_mock/WP_Mock.php:71
/home/travis/build/goblindegook/Syllables/vendor/10up/wp_mock/WP_Mock.php:101
/home/travis/build/goblindegook/Syllables/vendor/10up/wp_mock/WP_Mock/Tools/TestCase.php:37
```

Line 39 in `constant-mocks.php` tries to set the `OBJECT` constant in a case-insensitive manner:

```php
define( 'OBJECT', 'OBJECT', true );
```

This patch removes the third `define()` argument and applies a [workaround suggested by the HHVM team](http://hhvm.com/blog/3095/getting-wordpress-running-on-hhvm), which consists of declaring multiple variations of the `OBJECT` constant.