remove hhvm testing. test failed with ...

Using worker: worker-linux-2-1.bb.travis-ci.org:travis-linux-16
$ export DB=sqlite
git.1
$ git clone --depth=50 --branch=develop-2 git://github.com/auraphp/Aura.Sql.git auraphp/Aura.Sql
Cloning into 'auraphp/Aura.Sql'...
remote: Counting objects: 2620, done.
remote: Compressing objects: 100% (1131/1131), done.
remote: Total 2620 (delta 1307), reused 2415 (delta 1150)
Receiving objects: 100% (2620/2620), 586.87 KiB, done.
Resolving deltas: 100% (1307/1307), done.
$ cd auraphp/Aura.Sql
git.3
$ git checkout -qf b957cb1
$ phpenv global hhvm
$ php --version
HipHop VM v2.3.0 (rel)
Compiler: heads/HHVM-2.3-0-ga12fa4ac4dacbb45348a6ff2b3775f895883f934
Repo schema: 30349d0b2f68667d4e5a3fd291f2274c608f9e29
$ composer --version
Cannot open log file: /var/log/hhvm/error.log
HipHop Strict Warning: It is not safe to rely on the system's timezone settings. Please use the date.timezone setting, the TZ environment variable or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected 'UTC' for 'UTC/0.0/no DST' instead in phar://composer.phar/bin/../src/../src/Composer/Console/Application.php on line 65
Composer version 80499bb02418711b34bba59c1a6d8032429e5702 2013-12-06 12:32:19
before_script
$ cd tests
$ phpunit
Cannot open log file: /var/log/hhvm/error.log
PHPUnit 3.7.28 by Sebastian Bergmann.
Configuration read from /home/travis/build/auraphp/Aura.Sql/tests/phpunit.xml
.................E.......EE......HipHop Fatal error: Call to undefined method PDO::inTransaction from context Aura\Sql\ExtendedPdo in /home/travis/build/auraphp/Aura.Sql/src/ExtendedPdo.php on line 916
The command "phpunit" exited with 1.
Done. Your build exited with 1.

.travis.yml

@@ -5,7 +5,6 @@ php:
   - 5.3
   - 5.4
   - 5.5
-  - hhvm
 before_script:
   - cd tests
 script: phpunit