Auto merge of #6498 - bundler:colby/gem-util-compatability, r=colby-swandale
Auto merge of #6498 - bundler:colby/gem-util-compatability, r=colby-swandale

Fix calling undefined `Gem::Util.inflate` in RubyGemsIntegration for old RubyGems versions

### What was the end-user problem that led to this PR?

RubyGems integration is calling `Gem::Util.inflate` which is not available in all supported versions of RubyGems.

### What was your diagnosis of the problem?

See https://travis-ci.org/bundler/bundler/jobs/368919903

### What is your fix for the problem, implemented in this PR?

Check if `Gem::Util` is defined, and if not, fallback to using the old way calling `inflate` in RubyGems

(cherry picked from commit 1eb812c8bdc4de6b052e2262b5d3d3d19f90d8a7)

dilumnavanjana/bundler,mvz/bundler,mvz/bundler,bundler/bundler,bundler/bundler,dilumnavanjana/bundler