update travis for deployment on v3.1.1

.travis.yml

@@ -23,8 +23,8 @@ install:
   - tar -xzf xtensa-esp32-elf-linux64-1.22.0-80-g6c4433a-5.2.0.tar.gz
   # Make xtensa-esp32-elf available for all terminal sessions
   - export PATH=$PATH:$HOME/esp/xtensa-esp32-elf/bin
-  # Get ESP-IDF v3.3
-  - git clone --branch v3.3 --recursive https://github.com/espressif/esp-idf.git
+  # Get ESP-IDF v3.3.1
+  - git clone --branch v3.3.1 --recursive https://github.com/espressif/esp-idf.git
   # Set the path to ESP-IDF directory
   - export IDF_PATH=~/esp/esp-idf 
   # Install Required Python Packages