Reduce Ruby warnings
Problem
===
sassc-ruby displays warnings on load the gem if `-w` options is passed to ruby interpreter.
```console
$ ruby -w -rsassc -e ''
/home/pocke/.rbenv/versions/trunk/lib/ruby/gems/2.7.0/gems/ffi-1.11.1/lib/ffi/library.rb:275:
warning: method redefined; discarding old version
/home/pocke/.rbenv/versions/trunk/lib/ruby/gems/2.7.0/gems/ffi-1.11.1/lib/ffi/library.rb:275:
warning: method redefined; discarding old version
/home/pocke/.rbenv/versions/trunk/lib/ruby/gems/2.7.0/gems/ffi-1.11.1/lib/ffi/library.rb:275:
warning: method redefined; discarding old _make_data_context
/home/pocke/.rbenv/versions/trunk/lib/ruby/gems/2.7.0/gems/ffi-1.11.1/lib/ffi/library.rb:275:
warning: method redefined; discarding old _make_data_context
/home/pocke/.rbenv/versions/trunk/lib/ruby/gems/2.7.0/gems/ffi-1.11.1/lib/ffi/library.rb:275:
warning: method redefined; discarding old _context_get_included_files
/home/pocke/.rbenv/versions/trunk/lib/ruby/gems/2.7.0/gems/ffi-1.11.1/lib/ffi/library.rb:275:
warning: method redefined; discarding old _context_get_included_files
/home/pocke/.rbenv/versions/trunk/lib/ruby/gems/2.7.0/gems/ffi-1.11.1/lib/ffi/library.rb:275:
warning: method redefined; discarding old list_get_length
/home/pocke/.rbenv/versions/trunk/lib/ruby/gems/2.7.0/gems/ffi-1.11.1/lib/ffi/library.rb:275:
warning: method redefined; discarding old list_get_length
/home/pocke/.rbenv/versions/trunk/lib/ruby/gems/2.7.0/gems/ffi-1.11.1/lib/ffi/library.rb:275:
warning: method redefined; discarding old list_get_value
/home/pocke/.rbenv/versions/trunk/lib/ruby/gems/2.7.0/gems/ffi-1.11.1/lib/ffi/library.rb:275:
warning: method redefined; discarding old list_get_value
```
This pull request suppresses the warnings.
Note
===
sassc-ruby has other warnings in test code, but this pull request does not suppress the warnings. Because they does not affect users.
If you'd like to see the warning, you can execute `rake test` with `RUBYOPT=-w` environment variable.