Loosen opentracing constraint

[cptaffe]

For projects which use rack-tracer and grpc-opentracing, bundler cannot install because the opentracing gem constraints are incompatible (reproduced below). Since this gem is a dependency of grpc-opentracing, it has to be massaged before grpc-opentracing can be.
All tests pass on opentracing 0.5.

Resolving dependencies...
Bundler could not find compatible versions for gem "opentracing":
  In snapshot (Gemfile.lock):
    opentracing (= 0.5.0)

  In Gemfile:
    grpc-opentracing was resolved to 1.0.0, which depends on
      method-tracer (~> 1.1) was resolved to 1.1.2, which depends on
        opentracing (~> 0.3.1)

    rack-tracer was resolved to 0.9.0, which depends on
      opentracing (~> 0.4)

Running `bundle update` will rebuild your snapshot from scratch, using only
the gems in your Gemfile, which may resolve the conflict.