Bump image_optim from 0.26.3 to 0.26.4

[dependabot-preview[bot]]

Bumps image_optim from 0.26.3 to 0.26.4.

Changelog

*Sourced from [image_optim's changelog](https://github.com/toy/image_optim/blob/master/CHANGELOG.markdown).* > ## v0.26.4 (2019-05-23) > > * Enable frozen string literals [@​toy](https://github.com/toy)

Commits

- [`d4003e5`](https://github.com/toy/image_optim/commit/d4003e5f651d1a35f131fb0044552ba5d94ce7ec) v0.26.4 - [`75cade8`](https://github.com/toy/image_optim/commit/75cade83d2657933698f1df9a6efdb074173d7f8) changelog entry for enabling frozen string literals - [`6161167`](https://github.com/toy/image_optim/commit/61611674d78afa976fbe0540a9f06dc4752807af) adapt to frozen string literals - [`9aa7ea9`](https://github.com/toy/image_optim/commit/9aa7ea91446964dd3ac711e6a08a5d567f03d21c) rubocop 0.69.0 - [`7764963`](https://github.com/toy/image_optim/commit/77649638ddf6d33cdb1421a37bb95d6e610633df) forbid rspec-expectations 3.8.3 - [`fcf7ec4`](https://github.com/toy/image_optim/commit/fcf7ec436803793b995d84fdbe2a8b5f55306b2b) rubocop 0.68.1 - [`9cccecd`](https://github.com/toy/image_optim/commit/9cccecda1b844cd5bd0763e717431d30bbb16664) travis: ruby 2.6.3 - [`8c5f101`](https://github.com/toy/image_optim/commit/8c5f10128d52a7e3ef1d85ad7cdfaf335dad3690) travis: ruby 2.4.6 - [`7ddbb48`](https://github.com/toy/image_optim/commit/7ddbb484b0088c1d2d3048234adb317fb9fca459) travis: ruby 2.5.5 and 2.6.2 - [`c91f34f`](https://github.com/toy/image_optim/commit/c91f34f55b6e01c99bd4bf8df55fe21a58804b44) travis: don’t depend on rubocop with jruby - Additional commits viewable in [compare view](https://github.com/toy/image_optim/compare/v0.26.3...v0.26.4)

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[dependabot-preview[bot]]

Looks like image_optim is up-to-date now, so this is no longer needed.