Adding apt-get update as suggested by manual

Manual at: https://docs.travis-ci.com/user/installing-dependencies/

Signed-off-by: Simon Egli <simon.egli@bbv.ch>

.travis.yml

@@ -1,5 +1,6 @@
 language: cpp
 before_install:
+- sudo apt-get -qq update
 - sudo apt-get install -y python-serial srecord
 install:
 - wget https://github.com/GeorgeHahn/nodemcu-firmware/raw/travis/tools/esp-open-sdk.tar.gz -O tools/esp-open-sdk.tar.gz