Fix travis build

Author: Juanjo Alvarez

Makefile

@@ -1,9 +1,11 @@
 test-native-internal:
-	cd native/python_package/test; \
+	pip3 install --user native/dev_deps/python-pydetector/ || pip3 install --user pydetector-bblfsh
+	pip3 install --user git+https://github.com/python/mypy.git@0bb2d1680e8b9522108b38d203cb73021a617e64#egg=mypy-lang
+	cd native/python_package/test && \
 	python3 -m unittest discover
 
 build-native-internal:
-	cd native/python_package/; \
+	cd native/python_package/ && \
 	pip3 install -U --user .
 	cp native/sh/native.sh $(BUILD_PATH)/native;
 	chmod +x $(BUILD_PATH)/native