Raise Interrupt instead of StandardExit

This fixes a bug when using Ruby 1.8, since it has no definition of
StandardExit:

"uninitialized constant VimGolf::CLI::StandardExit (NameError)"

Since Interrupt and StandardExit executed the same code when caught, we
just replaced all occurences of StandardExit by Interrupt.

The bug was introduced in 103e5279281bb3c6deed76f7b9c6d1558d74565e