Clean up gemspec a bit

* remove useless %q
* align executables detection with test files
* replace `$\` with its human friendly alias `$RS`
- Better handling of magic encoding comment edge cases (fixes #33).
- Fix quantifier in magic encoding parser (refs #33).
- Don't allow destructive mutation of line cache in Source::Buffer.
- lexer.rl: "Option /^I/" is a method call (fixes #32).
Merge pull request #30 from bbatsov/gemspec

Clean up gemspec a bit
Replace split($RS) with split, as $RS is the default.
+ Add `dot' source map to (send) nodes (fixes #34).
+ Better location reporting for hashes with labels.
Uncomment test in test_lexer.rb.