[TASK] Add support for PHP 5.4 in Travis-CI build file

At the moment, the Travis-CI build file only supports PHP 5.3.
I`ve tested to run the same UnitTests for PHP 5.4 and it
works quite well:

http://travis-ci.org/#!/andygrunwald/TYPO3v4-Core/builds/1746689

Change-Id: I47559493900f56d038243bf13ab116455b4f49bb
Fixes: #38508
Releases: 6.0
Reviewed-on: http://review.typo3.org/12486
Reviewed-by: Philipp Gampe
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn

.travis.yml

 language: php
 php:
   - 5.3
+  - 5.4
 
 env:
   - DB=mysql