Enabled asset helpers in precompiled templates

What's Changed

Test against latest ruby and rails by @tagliala in https://github.com/slim-template/slim-rails/pull/161
Add erb to slim instructions in README by @MarkFChavez in https://github.com/slim-template/slim-rails/pull/163
Enable asset helpers in precompiled templates by @yasaichi in https://github.com/slim-template/slim-rails/pull/135
gemspec: Include fewer files. by @olleolleolle in https://github.com/slim-template/slim-rails/pull/172

@MarkFChavez made their first contribution in https://github.com/slim-template/slim-rails/pull/163
@yasaichi made their first contribution in https://github.com/slim-template/slim-rails/pull/135
@olleolleolle made their first contribution in https://github.com/slim-template/slim-rails/pull/172

Full Changelog: https://github.com/slim-template/slim-rails/compare/v3.2.0...v3.3.0