[TravisCI] install pip in osx builds

https://github.com/travis-ci/travis-ci/issues/2312

.travis.yml

@@ -80,6 +80,8 @@ install_osx: &install_osx
   - ls -l out
   - export PATH="$PATH:$TRAVIS_BUILD_DIR"
   - export HAXE_STD_PATH="$TRAVIS_BUILD_DIR/std"
+  # Install pip (used in lua test)
+  - curl https://bootstrap.pypa.io/get-pip.py -o - | sudo python
 
 matrix:
   include: