Remove ftw dependency which depends on Native modules

[aodhan-domhnaill]

See https://app.travis-ci.com/github/logstash-plugins/logstash-input-couchdb_changes/jobs/544759258

current directory:
/usr/share/logstash/vendor/jruby/lib/ruby/gems/shared/gems/http_parser.rb-0.8.0/ext/ruby_http_parser
make "DESTDIR="
sh: make: command not found

make failed, exit code 127

Gem files will remain installed in
/usr/share/logstash/vendor/jruby/lib/ruby/gems/shared/gems/http_parser.rb-0.8.0
for inspection.
Results logged to
/usr/share/logstash/vendor/jruby/lib/ruby/gems/shared/extensions/universal-java-11/2.5.0/http_parser.rb-0.8.0/gem_make.out

An error occurred while installing http_parser.rb (0.8.0), and Bundler cannot
continue.
Make sure that `gem install http_parser.rb -v '0.8.0' --source
'https://rubygems.org/'` succeeds before bundling.

In Gemfile:
  ftw was resolved to 0.0.49, which depends on
    http_parser.rb

Since we are using jruby, I do not think we will be able to build a native module.