Fix bundler_injection for environment that has a embedded ruby

Hello.

In environment with embedded ruby, such as td-agent, the `--gemfile` option may be unavailable.
It will be caused by `bundler_injection.rb` try to exec system's `bundle` command.
This PR is going to make it find `bundle` from current ruby's gems.

If this looks good to merge, there are two questions.

1. I couldn't find tests about `bundle_inject.rb`. Should I add it?
2. Should I make it will put some error message when it cannot find `bundle` command.

Thank you.