Pin salt-minion version to 2015.5.

2015.8.0 causes breakage on OS x and 2015.8.1, which fixes it, is not
yet available in homebrew.

Use 2015.5.6 for Ubuntu 12.04 and 2015.5.5 for OS X.

.travis/install_salt

@@ -1,15 +1,18 @@
 #!/usr/bin/env bash
 
+# Ensure that pinned versions match as closely as possible
+
 if [[ "${TRAVIS_OS_NAME}" == "linux" ]]; then
   printf "$0: installing salt for Linux\n"
-  # Travis uses Ubuntu 12.04
-  sudo add-apt-repository -y ppa:saltstack/salt
+  # Travis uses Ubuntu 12.04 (Precise Pangolin)
+  wget -O - https://repo.saltstack.com/apt/ubuntu/ubuntu12/latest/SALTSTACK-GPG-KEY.pub | sudo apt-key add -
+  printf 'deb http://repo.saltstack.com/apt/ubuntu/ubuntu12/2015.5 precise main\n' | sudo tee -a /etc/apt/sources.list >/dev/null
   sudo apt-get -y update
-  sudo apt-get -y install salt-minion
+  sudo apt-get -y install salt-minion=2015.5.6+ds-1
 elif [[ "${TRAVIS_OS_NAME}" == "osx" ]]; then
   printf "$0: installing salt for Mac OS X\n"
   brew update
-  brew install saltstack
+  brew install https://raw.githubusercontent.com/Homebrew/homebrew/86efec6695b019762505be440798c46d50ebd738/Library/Formula/saltstack.rb
 else
   printf >&2 "$0: environment variable TRAVIS_OS_NAME not set or set to unknown value\n"
   exit 1