Use curl instead of wget to avoid cert issues

wget throws cert errors when attempting to download the SaltStack repo
GPG key from repo.saltstack.com, even though ca-certificates is
up-to-date.

.travis/install_salt

@@ -5,7 +5,7 @@
 if [[ "${TRAVIS_OS_NAME}" == "linux" ]]; then
   printf "$0: installing salt for Linux\n"
   # Use Trusty (Ubuntu 14.04) on Travis
-  wget -O - https://repo.saltstack.com/apt/ubuntu/ubuntu14/latest/SALTSTACK-GPG-KEY.pub | sudo apt-key add -
+  curl https://repo.saltstack.com/apt/ubuntu/ubuntu14/latest/SALTSTACK-GPG-KEY.pub | sudo apt-key add -
   printf 'deb http://repo.saltstack.com/apt/ubuntu/ubuntu14/2015.5 trusty main\n' | sudo tee -a /etc/apt/sources.list >/dev/null
   sudo apt-get -y update
   sudo apt-get -y install salt-minion=2015.5.6+ds-1