Merge pull request go-redsync#61 from bancek/fix-extend-unlock

Fix Extend and Unlock

Merge pull request go-redsync#57 from NathanBaulch/master

Simplify recently added context support

Update README.md

Update README.md

Update README.md

Update README.md

Update .travis.yml

Update .travis.yml

Fix Valid function

Merge pull request go-redsync#42 from cwjenkins/feature/add_valid_met…

…hod_for_mutex

Providing a 'check' to ensure a lock is still valid prior to writing.