Fix travis build

Author: GuillaumeGomez

.travis.yml

@@ -52,7 +52,7 @@ addons:
 before_install:
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install gtk+3 cairo atk; fi
-  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:/opt/X11/lib/pkgconfig; fi
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:/opt/X11/lib/pkgconfig:/usr/local/opt/libffi/lib/pkgconfig; fi
   - if [[ "$ARM" == "1" ]]; then rustup target add armv7-unknown-linux-gnueabihf; fi
 script:
   - if [ "$TRAVIS_RUST_VERSION" == "beta" ]; then