Elastic beanstalk ebextensions/config : No package 'libffi' found

This is my config :

I only have 2 configs in .ebextensions : 01run.config and 02do.config

.ebextensions/01run.config

packages:
    yum:
        python27-devel: ''
        postgresql93-devel: ''
        pkgconfig: ''
        libffi-devel: ''
        libxslt-devel: ''
        libxml2-devel: ''

AND this has log , What else should I do ? Please help me! Thank you!!

  Downloading/unpacking cffi==0.8.6 (from -r /opt/python/ondeck/app/requirements.txt (line 8))
  Running setup.py (path:/opt/python/run/venv/build/cffi/setup.py) egg_info for package cffi
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found
    Package libffi was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libffi.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libffi' found