Fix problem with Duplicate declaration of Package[rake]

Apparently the puppet dashboard module already defines the rake
package. Change to only install rake and puppetlabs_spec_helper
on jenkins and slaves via Ruby gem.

Change-Id: I607b673620cb00dae72c3fd90e0dc3eb32504162
Closes-Bug: 1338334
Merge "Fix problem with Duplicate declaration of Package[rake]"