Relax rspec_junit_formatter version constrant

As currently implemented, I get this error when I try to do `bundle update`:

```
$ bundle update
Fetching gem metadata from https://rubygems.org/.......
Fetching additional metadata from https://rubygems.org/..
Resolving dependencies...
Bundler could not find compatible versions for gem "rspec":
  In Gemfile:
    rspec_junit_formatter (~> 0.1.6) ruby depends on
      rspec (~> 2.0) ruby

    chefspec (~> 4.0.1) ruby depends on
      rspec (3.0.0)
```