Include after_failure in travis to see log error

Signed-off-by: Rogerio Prado de Jesus <rogeriopradoj@gmail.com>
Run travis tests in PHP 5.2.17

As 5.2.17 is the last one of 5.2 branch, it would be ok to have this
tested as well despite of it isn't one of official supported version in
php.net nowadays.

In order to don't break completely our build, if it fails, 5.2.17 is
include in section `allow_failures` in Travis, keeping the build ok it
the newer definitions goes ok as well.

Signed-off-by: Rogerio Prado de Jesus <rogeriopradoj@gmail.com>
Merge pull request #234 from rogeriopradoj/travis-cat-log-after-failures

[travis] Cat log errors in case of failure and include 5.2.17 in travis build but as an allowed failure