Use tilt 1.4.0+ (fixes haml & UTF-8 issue #519)

Today, when I tried Padrino it failed to render haml page with my native characters in UTF-8 format.
I tried Sinatra and it didn't have problem.

I soon found issue #519. While https://github.com/padrino/padrino-framework/issues/519#issuecomment-4750542 worked for rendering, it broke rspec.

Looking at Gemspec.lock of my projects with Sinatra and Padrino I noticed that Sinatra had newer tilt.
So I tried updating tilt for Padrino. It worked - I could render the page that failed previously.

I ran tests and they all passes. Hope you'll merge this one.