Try to install newer fish

.travis.yml

@@ -12,6 +12,8 @@ script: make test-all
 install: script/bootstrap
 addons:
   apt:
+    sources:
+      - sourceline: 'ppa:fish-shell/release-2'
     packages:
       - tmux
       - zsh

script/ruby-test

@@ -26,6 +26,7 @@ if tmux -V; then
     git --version
     bash --version | head -1
     zsh --version
+    fish --version
     echo
   fi
   profile="all"