Python Couchbase CFFI - `AssertionError: backend.__version__ == __version__[:3])`

问题

Full error text:

No module named couchbase._libcouchbase
Traceback (most recent call last):
  File "app_main.py", line 75, in run_toplevel
  File "runtests.py", line 12, in <module>
    mod = __import__(t, globals(), locals(), ['suite'])
  File "/home/travis/build/ardydedase/pycouchbase/tests/test_pycouchbase.py", line 15, in <module>
    from pycouchbase.utils import *
  File "/home/travis/build/ardydedase/pycouchbase/pycouchbase/__init__.py", line 11, in <module>
    import couchbase_ffi as couchase
  File "/home/travis/virtualenv/pypy-2.5.0/site-packages/couchbase_ffi-0.2.0.0-py2.7-linux-x86_64.egg/couchbase_ffi/__init__.py", line 50, in <module>
    import couchbase_ffi._libcouchbase as _libcouchbase
  File "/home/travis/virtualenv/pypy-2.5.0/site-packages/couchbase_ffi-0.2.0.0-py2.7-linux-x86_64.egg/couchbase_ffi/_libcouchbase.py", line 3, in <module>
    from couchbase_ffi._cinit import get_handle
  File "/home/travis/virtualenv/pypy-2.5.0/site-packages/couchbase_ffi-0.2.0.0-py2.7-linux-x86_64.egg/couchbase_ffi/_cinit.py", line 13, in <module>
    ffi = FFI()
  File "/home/travis/virtualenv/pypy-2.5.0/site-packages/cffi/api.py", line 59, in __init__
    backend.__version__ == __version__[:3])
AssertionError

This only happens when I try to import couchbase_ffi in the PyPy build on Travis: https://travis-ci.org/ardydedase/pycouchbase/jobs/76043402#L1663

Details:

I have this in my requirements.txt for couchbase-python-cffi: