Make assets precompile symlink task compatible with Rails 3

[etripier]

@justin808 Per #564.

This allows react_on_rails to parse legacy manifest.yml documents to discover digest filenames when symlinking compiled assets.

---

This change is

[justin808]

@etripier this looks good. However, CI is not passing. Please investigate.

[coveralls]

Coverage increased (+0.06%) to 82.987% when pulling 19c5b01 on etripier:make-assets-precompile-symlink-task-compatible-with-rails-3 into ba16714 on shakacode:master.

[etripier]

@justin808 CI is passing now (had to reduce the complexity of my change a bit). Here's a diff without the whitespace: https://github.com/shakacode/react_on_rails/pull/566/files?w=1.

[justin808]

@etripier Please update the CHANGELOG.md.

See https://github.com/shakacode/react_on_rails/blob/master/CONTRIBUTING.md

[etripier]

@justin808 Updated the CHANGELOG. Looks like Capybara failed on a timeout error, however - is there a way to restart the CI checks?

[justin808]

@etripier There's a button on the bulid page.

[justin808]

@etripier Please rebase on top of master. I fixed the CI issue. We'll merge and release after that.

[etripier]

@justin808 Sorry for the wait - I rebased my commits. Thanks!

[coveralls]

Coverage increased (+0.06%) to 82.987% when pulling 2af61b4 on etripier:make-assets-precompile-symlink-task-compatible-with-rails-3 into bfa238a on shakacode:master.

[justin808]

Shipped in 6.1.2