Tighten dependency on Haml 3.1

haml-magic-translations currently does not support Haml 4.0.
Add support for the 'maruku' filter, behaving like 'markdown'
Explicitly use maruku to test Markdown translations

Otherwise, we might see different test results if rdiscount or peg_markdown are
available.
Adjust translated JavaScript strings in specs

It looks like Unicode characters are now always escaped in Haml output.
Version bump to 0.4.0
Regenerate gemspec for version 0.4.0