Add Travis build for PHP 5.3.3

As I have failed tests on my computer with PHP 5.3.3, I'd like to run them on Travis to see if it works.

```
gnutix@gnutix-debian:~/lamp/oss/geojson$ php -v
PHP 5.3.3-7+squeeze14 with Suhosin-Patch (cli) (built: Aug  6 2012 14:18:06) 
Copyright (c) 1997-2009 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2010 Zend Technologies
    with Xdebug v2.1.0, Copyright (c) 2002-2010, by Derick Rethans
    with Suhosin v0.9.32.1, Copyright (c) 2007-2010, by SektionEins GmbH
    
gnutix@gnutix-debian:~/lamp/oss/geojson$ bin/phpunit 
PHPUnit 3.7.28 by Sebastian Bergmann.

Configuration read from /home/gnutix/lamp/oss/geojson/phpunit.xml.dist

FF.........FF.............................FF...................  63 / 112 ( 56%)
.................................................

Time: 57 ms, Memory: 11.25Mb

There were 6 failures:

1) GeoJson\Tests\BoundingBoxTest::testIsJsonSerializable
Failed asserting that false is true.

/home/gnutix/lamp/oss/geojson/tests/GeoJson/Tests/BoundingBoxTest.php:11

2) GeoJson\Tests\BoundingBoxTest::testIsJsonUnserializable
Failed asserting that false is true.

/home/gnutix/lamp/oss/geojson/tests/GeoJson/Tests/BoundingBoxTest.php:16

3) GeoJson\Tests\CoordinateReferenceSystem\CoordinateReferenceSystemTest::testIsJsonSerializable
Failed asserting that false is true.

/home/gnutix/lamp/oss/geojson/tests/GeoJson/Tests/CoordinateReferenceSystem/CoordinateReferenceSystemTest.php:14

4) GeoJson\Tests\CoordinateReferenceSystem\CoordinateReferenceSystemTest::testIsJsonUnserializable
Failed asserting that false is true.

/home/gnutix/lamp/oss/geojson/tests/GeoJson/Tests/CoordinateReferenceSystem/CoordinateReferenceSystemTest.php:22

5) GeoJson\Tests\GeoJsonTest::testIsJsonSerializable
Failed asserting that false is true.

/home/gnutix/lamp/oss/geojson/tests/GeoJson/Tests/GeoJsonTest.php:11

6) GeoJson\Tests\GeoJsonTest::testIsJsonUnserializable
Failed asserting that false is true.

/home/gnutix/lamp/oss/geojson/tests/GeoJson/Tests/GeoJsonTest.php:16

FAILURES!
Tests: 112, Assertions: 418, Failures: 6.
```