Failed to convert a form with java7, python3 environment

[rhythmize]

Encountered issue while trying to convert a form. Updating to Java 8 fixed the issue. Error thrown was not intuitive, should be more explicit. Nowhere it was mentioned that java 7 won't work.

Traceback (most recent call last):
  File "pyxform/xls2xform.py", line 146, in <module>
    main_cli()
  File "pyxform/xls2xform.py", line 137, in main_cli
    enketo=args.enketo_validate)
  File "pyxform/xls2xform.py", line 27, in xls2xform_convert
    warnings=warnings, enketo=enketo)
  File "/home/rhythm/Desktop/odk/pyxform/pyxform/survey.py", line 820, in print_xform_to_file
    warnings.extend(odk_validate.check_xform(path))
  File "/home/rhythm/Desktop/odk/pyxform/pyxform/validators/odk_validate/__init__.py", line 76, in check_xform
    b'ODK Validate Errors:\n' + ErrorCleaner.odk_validate(stderr).encode('utf-8'))
pyxform.validators.odk_validate.ODKValidateError: b'ODK Validate Errors:\nException in thread "main" java.lang.UnsupportedClassVersionError: org${XFormParseException} : Unsupported major.minor version 52.0'

So, I propose that README should be updated along with error message.
I wish work on this issue.