Fix bug in release script due to typo.

* Fix collection typo
* Create shared command to setup 2.11 builds
* Alter snapshot detection to allow more flexible milestone versioning.
* Remove cusotm isSnapshot key for new sbt 0.13.2 isSnapshot key.
Fix bug in release script due to typo.

* Fix collection typo
* Create shared command to setup 2.11 builds
* Alter snapshot detection to allow more flexible milestone versioning.
* Remove cusotm isSnapshot key for new sbt 0.13.2 isSnapshot key.
Also publish completions library for 2.11.
merge with master
Merge pull request #1350 from sbt/wip/fix-release-scripts

Fix bug in release script due to typo.