This limits 1.8.7 to use `activesupport-v3.2.21`

Versions greater than 4.0 dropped 1.8.7 support but it appears the
Travis build environment just updated so this appeared.
Merge pull request #3353 from fog/enforce_18_activesupport_dependency

This limits 1.8.7 to use `activesupport-v3.2.21`