Add OSX ci.

.travis.yml

@@ -1,7 +1,17 @@
 sudo: required
 before_install:
-    - sudo add-apt-repository -y ppa:fish-shell/release-2
-    - sudo apt-get update
-    - sudo apt-get -y install fish
+    - >
+        if [[ $TRAVIS_OS_NAME = "linux" ]]; then
+          sudo add-apt-repository -y ppa:fish-shell/release-2
+          sudo apt-get update
+          sudo apt-get -y install fish
+        else
+          brew update
+          brew install fish
+        fi
+    - fish --version
 script:
     - make && fish -c "fisher install fishtape; fishtape test/*.fish"
+os:
+    - linux
+    - osx