Replace installing python3.6 command on Makefile (#41)

* Modify Makefile

* Modify .travis.yml

* Replace make command on .travis.yml

Author: MrSyee

.travis.yml

@@ -6,6 +6,7 @@ python:
     - "2.7"
 
 install:
+    - make py36
     - make dep
     - make dev
 

Makefile

@@ -7,12 +7,16 @@ format:
 
 dev:
 	pip install -r scripts/requirements-dev.txt
-	sudo add-apt-repository -y ppa:deadsnakes/ppa
-	sudo apt-get update
-	sudo apt-get install -y python3.6
-	sudo apt-get install -y python3-pip
-	python3.6 -m pip install black
+	python3.6 -m pip install black  # required python3.6 & python3-pip
 	pre-commit install
 
 dep:
 	pip install -r scripts/requirements.txt
+
+py36:
+	# install python3.6 for black (only ubuntu)
+	pip install -r scripts/requirements-dev.txt
+	sudo add-apt-repository -y ppa:deadsnakes/ppa
+	sudo apt-get update
+	sudo apt-get install -y python3.6
+	sudo apt-get install -y python3-pip