misc/ruby-bundler: update to 1.17.3

pkgsrc change: update HOMEPAGE.

1.17.3 (2018-12-27)

Bugfixes:

* Fix a Bundler error when installing gems on old versions of RubyGems (#6839,
  @colby-swandale)
* Fix a rare issue where Bundler was removing itself after a bundle clean
  (#6829, @colby-swandale) Documentation:
Add entry for the bundle remove command to the main Bundler manual page

(taca)