Restore cache updating when using --local.

Revert "Merge pull request #2509 from TimMoore/2494-local-implies-no-cache"

This reverts commit ddd2c83e24863c10b0094941cc0070737864e613, reversing
changes made to ac9092b47d85dba6780d4a4f49630b075e0b5270.

Conflicts:
	CHANGELOG.md
	lib/bundler/cli.rb

Closes #2791.