- Cleanup .travis.yml
  - Loops over std/no_std targets
  - Remove deprecated/useless keys
  - No more `cd`, we just use `--package`.
- Improve tests
  - Main `getrandom` tests are now unit test modules instead of
    integration tests, making the code cleaner.
  - The custom RNG crates now use this common module as part of their
    integration tests.
  - No more weird test-only features needed to get the crate to build.

Signed-off-by: Joe Richey <joerichey@google.com>