Fix package future dependency.

.travis.yml

@@ -1,6 +1,7 @@
 language: cpp
 before_install:
-- sudo apt-get install -y python-serial srecord
+- sudo apt-get install -y python-serial srecord python-pip
+- sudo pip install future
 install:
 - wget https://github.com/GeorgeHahn/nodemcu-firmware/raw/travis/tools/esp-open-sdk.tar.gz -O tools/esp-open-sdk.tar.gz
 - tar -zxvf tools/esp-open-sdk.tar.gz

README.md

@@ -16,6 +16,9 @@ home: [nodemcu.com](http://www.nodemcu.com)<br />
 bbs: [Chinese bbs](http://bbs.nodemcu.com)<br />
 Tencent QQ group: 309957875<br />
 
+# Attention
+If you want to use esptool.py or make, please `sudo pip install future` first.
+
 # Summary
 - Easy to access wireless router
 - Based on Lua 5.1.4 (without *io, math, debug, os* module.)