Version dependencies / Release 0.4.3

[mwear]

While building the v0.4.2 gem release there were several warnings regarding open ended dependencies. For example:

WARNING:  open-ended dependency on simplecov (>= 0, development) is not recommended
  if simplecov is semantically versioned, use:
    add_development_dependency 'simplecov', '~> 0'
WARNING:  See http://guides.rubygems.org/specification-reference/ for help

My first attempt to fix this issue was e588d96 which was released as 0.4.2. Unfortunately, the bundler versioning broke the [build](https://travis-ci.org/opentracing/opentracing-ruby/builds/444695333. While there isn't technically anything wrong with the ruby code, the officially released gem should have a passing build.

I think it's probably overkill to yank 0.4.2, but I'd like to release this as 0.4.3. Do these changes look ok and does the release plan sound reasonable to everyone?

cc: @indrekj @itsderek23