Use Rake v10.1 for development because v10.2 requires Ruby19 or higher

Rake version `10.2.0` was released yesterday (March 24th), and requires Ruby19 as the minimum required ruby version.  Since `ffaker` is being tested on Ruby18 on Travis-CI, you have to adjust your that line in your  gemspec to use Rake version `10.1.x` or all Ruby18 tests will error out (because that version of Rake can't be installed).

See [here for an example of this type of failure](https://travis-ci.org/EmmanuelOga/ffaker/jobs/21530514#L38).