Attempted workaround for macOS SCons build failure

Homebrew should really install libffi implicitly when we install anything
that depends on it. But let's try to work around the problem by installing
it explicitly.

utils/travis/steps/install.sh

@@ -23,7 +23,7 @@ if [ "$TRAVIS_OS_NAME" = "osx" ]; then
         brew install ccache
         travis_wait ./projectfiles/Xcode/Fix_Xcode_Dependencies
     else
-        brew install scons cairo pango moreutils sdl2_image sdl2_ttf sdl2_mixer glew ccache
+        brew install scons libffi cairo pango moreutils sdl2_image sdl2_ttf sdl2_mixer glew ccache
     fi
 else
     if [ "$NLS" != "true" ]; then