install swig in brew

.travis.yml

@@ -27,6 +27,7 @@ addons:
 
 before_install:
 - git submodule update --init --recursive
+- if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install swig; fi
 # - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then docker run -dit --name emscripten -v $(pwd):/src trzeci/emscripten:sdk-incoming-64bit bash ; fi
 # - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then mkdir -p /usr/local/musl/lib && cp /usr/lib/x86_64-linux-musl/libc.a /usr/local/musl/lib/libc.a ;fi