Problem with ant.
More specifically add the following in the beginning of your classpath:
%ANT_HOME%\lib\ant.jar
%ANT_HOME%\lib\ant-nodeps.jar
%ANT_HOME%\lib\ant-launcher.jar
I had the same problem ?and I added these three and it worked.
I got the idea from an IntelliJ thread:
http://support.jetbrains.com/kb/entry.jspa?externalID=59&categoryID=27