Added futures to requirements if python_version < 3 (#25)
Added futures to requirements if python_version < 3 (#25)

Fixes #23
yurishkuro/opentracing-tutorial,yurishkuro/opentracing-tutorial,yurishkuro/opentracing-tutorial,yurishkuro/opentracing-tutorial,yurishkuro/opentracing-tutorial,yurishkuro/opentracing-tutorial