I just upgraded from 1.415 to 1.418 and all of my builds now fail. Previously, the "invoke ant" build step ran builds like this:

 
08:50:05  [project] $ cmd.exe /C '"c:\jenkins\tools\Ant-1.8.2\bin\ant.bat jars && exit %%ERRORLEVEL%%"'
08:50:05  Buildfile: c:\jenkins\workspace\project\build.xml

After upgrading, the output looks like this:

 
09:25:05  [project] $ cmd.exe /C '"ant.bat && exit %%ERRORLEVEL%%"'
09:25:06  'ant.bat' is not recognized as an internal or external command,
09:25:06  operable program or batch file.
09:25:06  Build step 'Invoke Ant' marked build as failure

As you can see it's not using the version installed by Jenkins. I've downgraded build by build all the way back to 1.415, which fixes the problem. Along the way I noticed that between 1.415 and 1.416 the "ant version" dropdown in the Invoke Ant build step disappeared. This seems like it would cause the plugin to always use the "default" setting