Bump image_optim from 0.26.4 to 0.26.5

Bumps image_optim from 0.26.4 to 0.26.5.

Changelog

Sourced from image_optim's changelog.

v0.26.5 (2019-07-14)

• Remove deprecated rubyforge_project attribute from gemspec rubygems/rubygems#2436 @​toy

Commits

• 74a4e83 v0.26.5
• 3e9563d add changelog entry for removing deprecated rubyforge_project attribute from ...
• a4af287 remove deprecated rubyforge_project attribute from gemspec
• 20cb353 Revert "forbid rspec-expectations 3.8.3"
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