CI: Move to ESP-IDF 4.0 Release Branch

Move to the official ESP-IDF 4.0 release branch.

Author: sieren

.travis.yml

@@ -31,7 +31,7 @@ install:
   # Get ESP-IDF from github
   - git clone --recursive https://github.com/espressif/esp-idf.git
   - cd esp-idf
-  - git checkout 02f2e63662d0314b689bb4925f5d8efa08cfe033
+  - git checkout release/v4.0
   - git submodule update --init --recursive
   - cd ~
   - python -m pip install --user -r /home/travis/esp/esp-idf/requirements.txt