Update .travis.yml

Author: vinay0410

.travis.yml

@@ -30,6 +30,7 @@ before_install:
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then wget https://www.python.org/ftp/python/2.7.15/python-2.7.15-macosx10.6.pkg ; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then sudo installer -pkg python-2.7.15-macosx10.6.pkg -target / ; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then export PATH="/Library/Frameworks/Python.framework/Versions/2.7/bin:${PATH}" ; fi
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then sudo apt update ; fi
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then sudo apt install -y rapidjson-dev ; fi
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then sudo apt install -y libboost-dev libboost-filesystem-dev libboost-system-dev libboost-program-options-dev ; fi
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then sudo apt install -y libgoogle-glog-dev libyaml-cpp-dev qt5-default libqt5svg5-dev libqt5opengl5-dev ; fi