Replace Rake with Make

Replaces rake tasks with tidier and marginally faster make targets, while removing dependency on the Ruby toolchain for all tasks excepting `ci` and `upload`.

**Note:** `spec` is implicitly dependent on Ruby because of CocoaPods, but generally gets a speed bump by being run directly once pods are configured. 

Some tasks, like `version`, are now significantly simpler as targets since a lot of text escaping is avoided, making them easier to change later as needed.

## Changes
- Rewrite Rakefile as Makefile; the rake tasks were largely piping to the shell already
- Use `make ci` for travis builds
- Add `scripts` directory for extraneous bits, like uploading latest build to S3, which remains simpler in Ruby since the `aws-s3` gem handles authentication and configuring policy grants