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.

By adding lexical binding as a default option for the file, the problem
is seamlessly solved.
Merge pull request #11 from rranelli/master

Use lexical binding as default