version 2.6.0rc2

Merge pull request slackapi#687 from seratch/v2.6.0rc1-release-note

Add v2.6.0rc1 release note

Merge pull request slackapi#578 from stevengill/master

2.5.0

2.4.0 (slackapi#572)

Fixing recent regressions around sessions and views.publish Web API m…

…ethods

Adding new View APIs

v2.2.1 (slackapi#532)

* v2.2.1

Adding new API's & Fixing Bugs

New message builder and bug fixes

Fixing slackapi#422