Pin MDM gem to 0.17.x

Several incompatible 0.17 MDM versions were released that required us to
pin MDM in framework.  The incompatible versions have been yanked from
RubyGems.  This restores the original version range for MDM.

For users who respect our Gemfile.lock file, this is a no-op.

See #3495.

This reverts commit 8f39590f0f56987acb1246604f956f70191add5f.

Reported by @ZeroChaos