Disable fish_title on interactive tests

This was apparently what was interfering with the tests working on
Linux.

Tweak .travis.yml to install expect so the tests run on Linux.

tests/interactive.config

@@ -15,6 +15,9 @@ end
 
 set -g fish_greeting ''
 
+function fish_title
+end
+
 function _quote
     echo \'(echo $argv[1] | sed -e 's/\'/\\\\\'/g')\'
 end