Fix broken nginx PPA in Travis install script

Author: acrellin

.travis/travis_install.sh

@@ -8,7 +8,7 @@ section "install.base.requirements"
 firefox --version
 
 # Install v1.7 or newer of nginx to support 'if' statement for logging
-sudo apt-add-repository -y ppa:nginx/development
+sudo apt-add-repository -y ppa:nginx/stable
 sudo apt update
 sudo apt install -y nginx