improve gemspec

[noraj]

• fix the broken relative import
• remove git dependency (see https://stackoverflow.com/questions/11873294/determining-the-gems-list-of-files-for-the-specification)
• use bundler 2.x rather than 1.x

[aetherknight]

hey @noraj your changes look good at a glance, but they are causing CI to fail. Could you take a look at Travis's output to see if you can fix it? Thanks!

[noraj]

@aetherknight I can't. The CI failed because it was not able to download bundler 2.x even if bundle 2.0.0, 2.0.1, and 2.0.2 exist.
https://rubygems.org/gems/bundler
I'm don't know travis because I only use Gitlab-CI that can basically use every docker images.
You travis is configured to use ruby 2.2, 2.3 and 2.4 and no 2.5 and 2.6, maybe the old rubies have an old version of the rubygems gem or something.