running tests on linux and osx

.travis.yml

 language: c
 
+os:
+  - linux
+  - osx
+
 env:
-  - FISH_PPA=nightly-master
+  - FISH_PPA=nightly-master BREW_OPTIONS=--HEAD
 
 before_install:
   - script/bootstrap.sh

script/bootstrap.sh

 #!/usr/bin/env bash
-ppa=ppa:fish-shell/${FISH_PPA:-nightly-master}
-sudo add-apt-repository -y $ppa
-sudo apt-get update
-sudo apt-get -y install fish
+
+if [ "$TRAVIS_OS_NAME" = "linux" ]; then
+  ppa=ppa:fish-shell/$FISH_PPA
+  sudo add-apt-repository -y $ppa
+  sudo apt-get update
+  sudo apt-get -y install fish
+else
+  brew install fish $BREW_OPTIONS
+fi
+
 curl -L https://github.com/$TRAVIS_REPO_SLUG/raw/$TRAVIS_BRANCH/tools/install.fish | fish

script/run-tests.fish

 #!/usr/bin/env fish
 
-# When running on CI, run tests from the correct branch
-if set -q TRAVIS_BRANCH
-  command git checkout $TRAVIS_BRANCH
-end
-
 set -l result 0
 for test in (find * -type f -print | grep "spec.fish")
   fish $test $argv