I'm afraid that Subversion can only be integrated with JIRA Server though a plugin connection, but in Cloud platform we suggest using Fisheye, and hosting is on a standalone/self hosted server: http://www.atlassian.com/software/fisheye/overview
We also recommend trying Git in combination with our OnDemand Git hosting service - Bitbucket: https://bitbucket.org/
I have also found this plugin below, but I did not tested its integration with JIRA Cloud, feel free to have a look:
I hope that help!
Cheers, Rodrigo Silva
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