Update to continuation-local-storage@2.5.1

Resolves issue #71 ("AssertionError: context not currently entered; can't exit" on all http requests). 

2.5.1 is the latest release.

I am seeing a couple failing tests (e.g. test/integration/instrumentation-redis.tap.js) but I also see the same failures when I run `npm test` in master so I do not believe this update is an issue.