Install ruby right before "Installing the last dependencies"

[Lastorder-DC]

By executing RUBY_CONFIGURE_OPTS=--with-jemalloc rbenv install at mastodon source root directory, It will read from .ruby-version and Install proper version for mastodon.

No need to Install ruby before checking out source.

Pros]

1. No need to update document every time ruby version is updated(since It will read from .ruby-version)
2. If someone decided to use fork instead of mastodon, they no longer install outdated version if fork uses newer ruby version than mastodon.

Cons]

1. Nothing so far(ruby not needed at setting up db part)