Revert Thor dependency to 0.15.2 for TorqueBox 2.2.0 compatibility

When using TorqueBox 2.2, Bundler will not install the latest Padrino from GitHub with the updated Thor dependency. It clashes with TorqueBox's Thor version during `bundle install`.

Since the TB Bundler dependency is in a generated file created by a Maven build, I've reverted the version in padrino-core for testing with TorqueBox 2.2. Quick tests seem to be working, but I haven't generated everything. If Thor 0.16.x isn't required then this makes TB easier to work with.