Update opentracing dependency to v0.5.0

[michal-granec]

Hi,

I really appreciate this gem and I would like to update this gem and its feature set to fully support the new version of opentracing-ruby (v0.5.0). There are some changes in API which needs to be covered (like Scope, ScopeManager, Tracer#start_active_span, Span#log_kv, Tracer@start_span now accepts block...)

I will create PR in a few days.

[michal-granec]

I have create PR #20, so @iaintshine if you have time please take a look at the changes.

[michal-granec]

@iaintshine what do you think? Could this be merged?

I have used this code in production and there were no issues.