payment: remove PaymentMethodPlugin#getValueString

See https://github.com/killbill/killbill/commit/f010f28527fae2c0099492d35945bd1ae2f127d6.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
pom.xml: update to killbill-oss-parent 0.2.4

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
[maven-release-plugin] prepare release killbill-api-0.3.0