Use lexical binding as default

The `recompile` command won't work since the definitions of `file-name`
in `rubocop--file-command` (line 122) and `directory` in
`rubocop--dir-command` (line 86) won't be available in the lambdas given
to the `compilation-start` function, since Emacs does not use lexical scope by 
default.

By adding lexical binding as a default option for the file, the problem
is seamlessly solved (apparently).

Although I can sort of `recompile` by repeating the rubocop command with the
provided keybindings, I rather have the ability to just press <kbd>g</kbd> in
the compilation buffer or use my custom recompile binding.