TravisCI fix

Author: Daniel Reece Livingston

.travis.yml

@@ -15,7 +15,7 @@ branches:
 # Install dependencies for various OS'
 before_install:
   - if [ $TRAVIS_OS_NAME = windows ]; then choco install make cmake; fi
-  - if [ $TRAVIS_OS_NAME = linux ];   then sudo apt-get -y -f install gfortran libz-dev m4 bison python3 cmake && sudo apt-get upgrade cmake; fi
+  - if [ $TRAVIS_OS_NAME = linux ];   then sudo apt-get -y -f --allow-unauthenticated install gfortran libz-dev m4 bison python3 cmake && sudo apt-get upgrade cmake; fi
   - if [ $TRAVIS_OS_NAME = osx ];     then brew update && brew install gcc; fi
   - export FC=$(which gfortran);      echo $FC
   - export CC=$(which gcc);           echo $CC