Obey program suffix when invoking bundler

If ruby is configured using --program-suffix or --program-prefix the
rake tasks will not be able to launch the correct bundler executable (as
"bundler" may not exist).

This uses Gem.default_exec_format to choose the correct "bundler" to
invoke in the rake task based on how Ruby was configured.
Merge pull request #324 from drbrain/obey_program_suffix

Obey program suffix when invoking bundler