require yard and rspec only during dev

## motivation

Some tools, like briar, use `bundle exec` at _runtime_ to call various calabash gem take tasks.  Specifically, briar calls `$ rake build_server` as part of its automated workflow.

We don't want to require these tools include `rspec` and `yard` as runtime dependencies.

This pull requests wraps the `rspec` and `yard` require statements in a begin/rescue block that emits a warning when these gems are not part of the bundle.