Add fish to Travis build

.travis.yml

@@ -5,8 +5,7 @@ script:
   - ./lint.sh
 
 before_install:
-  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update; fi
-  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install shellcheck; fi
+  - ./prepare-travis.sh
 
 before_script:
   - git clone https://github.com/sstephenson/bats.git /tmp/bats

prepare-travis.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+
+# Install dependencies needed for build on Travis CI
+
+# Shellcheck is used by the linting script
+# Fish is needed for the tests for asdf.fish
+
+if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
+  brew update;
+  brew install shellcheck;
+
+  brew update
+  brew instal fish
+elif [[ "$TRAVIS_OS_NAME" = "linux" ]]; then
+  ${FISH_PPA:="nightly-master"}
+  PPA="ppa:fish-shell/$FISH_PPA"
+
+  sudo add-apt-repository -y "$PPA"
+  sudo apt-get update
+  sudo apt-get -y install fish
+fi