Remove unused SassContext module

It was removed here https://github.com/rails/sass-rails/commit/d357b0194015b05bf54f25ab7be6d6add5a91d1f
Merge pull request #147 from frodsan/remove_unused_module

Remove unused SassContext module
Remove sfl. Master does not support Ruby 1.8.
fix CHANGELOG format [ci skip]
copy edits on CHANGELOG [ci skip]
update CHANGELOG. closes #102 [ci skip]
Use .md extension for markdown files [ci skip]
s/ruby on rails/Ruby on Rails [ci skip]
update gemspec
rails -> Rails [ci skip]
Update README: config.sass.style is not supported - closes #103 [ci skip]
fix typo [ci skip]
add 339529f9 to the CHANGELOG [ci skip]