ENV[SWAGGER_BASE_PATH]

Allow tokens like ENV[SWAGGER_BASE_PATH] in the swagger JSON files to be replaced dynamically using environment variables.
https://www.pivotaltracker.com/story/show/79888872
How about changing the description to clarify what the method does now?

You should also add a test for the case when `basePath` is present in the input.

So e.g.

```ruby
  describe "overwrite_base_path" do
    it "adds basePath" do
      @opts = {overwrite_base_path: "baz"}
      result = overwrite_base_path("{\"foo\":\"bar\"}")
      expect(result).to eq("{\"foo\":\"bar\",\"basePath\":\"baz\"}")
    end
    it "overwrites basePath" do
      @opts = {overwrite_base_path: "baz"}
      result = overwrite_base_path("{\"basePath\":\"bar\"}")
      expect(result).to eq("{\"basePath\":\"baz\"}")
    end
  end
```