Update opentracing dependency to 0.5.0

[michal-granec]

I have updated OpenTracing dependencies to 0.5.0.

OpenTracing API has changed since version 0.3.2 - you can check Git diff but these are the most notable changes:

• Tracer#start_active_span - return Test::Scope and adds this scope to ScopeManager's stack
• Tracer#start_span can now accepts block
• Addition of Test::ScopeManager - scope manager keeps track of active span (with stack) tracer can access it with Tracer#scope_manager
• Addition of Test::Scope - scope encapsulate span
• Tracer#active_span - now returns active span - top of ScopeStack
• Span#log is now depracated in favour of Span#log_kv

I have added some useful public method/accessors to each new class. and also references attribute to Test::Span to be compatible with API.

These new changes should not break the basic idea of this tracer to record all spans in memory. You can now also inspect the current scope stack of the tracer.

The new parts are heavily influenced by Jaeger ruby client.

Please take a look if the tests are sufficient 📗

Refs #19