chore(travis): use ppa to install LuaJit-2.0.1 on Ubuntu
test(tests/lean): ExistsElim test

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/parser): make sure Lean passes all tests when being compiled with the readline library

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(tests/kernel/free_vars): reduce example stack size consumption

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>