added php ini configuration for travis
added memcache and services
added test for increment return type
try to fixed memcache::increment return string error
output memcache extension version for debug
try to fix memcache::increment method return string when version < 3.0.3
added compatible logic for memcache
enabled apc in cli
drop env configuation
fixed typo
try to fix travis apc test error: delete apc cache when expire < 0
removed apc extension on PHP 5.5, ref https://github.com/symfony/symfony/pull/7663/files
added redis install
changed serializer type to php for travis testing
used PHP serialize as default serializer instead of igbinary, for igbinary is not build in extension
Merge pull request #111 from twinh/feature/travis-php-extension

Enabled travis php extension for testing, close #109