Relax bundler dev dependency to allow bundler v2.0.0.pre2

[walf443]

ruby 2.6.0-rc1 was released.
https://www.ruby-lang.org/en/news/2018/12/06/ruby-2-6-0-rc1-released/

it seems to bundles bundler v2.0.0.pre2 https://github.com/bundler/bundler/releases/tag/v2.0.0.pre.2

I'd like to test rubocop with v2.6.0-rc1. But without this changes, it got following errors.

$ bunlde install
NOTE: Gem::Specification#has_rdoc= is deprecated with no replacement. It will be removed on or after 2018-12-01.
Gem::Specification#has_rdoc= called from /Users/yoshimin/src/github.com/whitequark/parser/parser.gemspec:15.
Fetching gem metadata from https://rubygems.org/.............
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Bundler could not find compatible versions for gem "bundler":
  In Gemfile:
    bundler (~> 1.15)

  Current Bundler version:
    bundler (2.0.0)
This Gemfile requires a different version of Bundler.
Perhaps you need to update Bundler by running `gem install bundler`?

Could not find gem 'bundler (~> 1.15)' in any of the relevant sources:
  the local ruby installation

[whitequark]

The changelog is autogenerated from commit messages, see CONTRIBUTING.md.

[walf443]

Oh. I see.

[iliabylich]

@walf443 Could you revert this change please?

[walf443]

@iliabylich I'm sorry to reply slowly. I fixed it.

[walf443]

Note: running on ruby 2.6.0-rc1 is failing by this issue ruby/racc#98

[whitequark]

It looks like newer bundler obsoletes every Ruby version prior to 2.3. You need to take these out of the Travis build matrix as well.

[walf443]

It may be #543 was cherry-picked this commit and it merged. so this issue should be closed. thanks.