[JENKINS-17680] Upgrade the configuration < 1.26 automatically.
[FIXED JENKINS-17680] Update a test for configuration-upgrading.
[JENKINS-17680] Make minimized diffs.
[JENKINS-17680] use @Initializer instead of ItemListener.
[JENKINS-17680] Upgrade is applied also to MatrixProject.
[JENKINS-17680] Added a test for verifying upgrading on building works.
[JENKINS-17680] Changed a confusiong method name.
Merge pull request #23 from ikedam/feature/JENKINS-17680-3

[JENKINS-17680] Upgrade configurations from version 1.25