Improve behaviour when posting to a shutdown thread pool

If an executor has a handle_overflow method defined, use that in the case where we post to an executor which is not running. If no handle_overflow method is defined (e.g. for the single-thread executor), preserve the current behaviour of returning false.

Fixes Github issue #192.
Make tests tolerate varied shutdown behaviors
Use the new build env on Travis

faster better stronger
Merge pull request #199 from rkday/post_to_shutdown

Improve behaviour when posting to a shutdown thread pool
Merge pull request #200 from joshk/patch-1

Use the new build env on Travis
Unify overflow/fallback handling between all four Executors
Rename 'overflow_policy' to 'fallback_policy'.

This reflects its broader scope (e.g. deciding how to handle tasks after shutdown).
Fixes from testing under JRuby
Merge pull request #201 from rkday/fallback_handling

Posting to a shutdown thread pool - JRuby consistency and better naming
Deprecation warnings on thread pools when :overflow_policy is used.
Updated changelog with post-0.7.1 updates.
Merge pull request #202 from ruby-concurrency/refactor/changelog-and-warnings

Warnings when using deprecated :overflow_policy thread pool option
Fixed a test I broke with overzealous find/replace.
Merge branch 'master' into promise-all