Currently there are many gotchas with warbler's bundling that require user awareness. We can do much better to "do the right thing" by default, by setting opinionated bundler config that is baked into the warbled application. This change

• bakes in a .bundle/config to the application, rather than relying on env vars, to ensure the strongest level of enforcement of isolation that will otherwise break the warbled jar/war
• ensures the following settings
  • BUNDLE_VERSION = system - ensure that bundler never tries to switch versions; and clarify that bundler will always run from the given jruby version's bundler.
  • BUNDLE_FROZEN = true - invert the previous default. Frozen should be the default, for both security and correctness. Users can still opt-out, if we have missed some case.
  • BUNDLE_PATH__SYSTEM = true - not strictly necessary, but clarifies that the jars/wars Warbler creates put all the gems onto what is effectively the system path at runtime.
  • BUNDLE_AUTO_INSTALL = false - enforce that installation of gems at runtime is impossible. Doubt anyone opts-in for this, but clarify that it won't work, and goes against the design goals of warbler.

Also cleans up related bugs for modern bundler

• properly excludes default gem stubs from being included, which for bundler can cause CorruptBundlerInstallError if there is any version mismatch.
• avoid duplicate gems when bundling relative path dependencies (Duplicate files with Gem dependency using :path #465)
• remove unnecessary legacy :warbler_excluded workaround for bundler issue handling excluded git specs (from warbler 1.3.1 - git gems not loaded #42) Git specs may still have some other problems however.
• correct support for gem_excludes within bundler git specs. (config.gem_excludes not working for bundle/git gems #331) Git specs may still have some other problems however.