Fix license link

`Logger#with_level` was recently added to enable configuring a
`Logger`'s level for the duration of a block. However, the configured
level is always tied to the currently running `Fiber`, which is not
always ideal in applications that mix `Thread`s and `Fiber`s.

For example, Active Support has provided a similar feature
(`ActiveSupport::Logger#log_at`) which, depending on configuration, can
be isolated to either `Thread`s or `Fiber`s.

This commit enables subclasses of `Logger` to customize the level
isolation. Ideally, it will enable replacing most of Active Support's
`#log_at`, since both methods end up serving the same purpose.

update license files in rdoc task

include license texts in gem