Merge pull request #40 from UseAllFive/to_s-fix

Add to_s to fix error when using stages.
Merge pull request #39 from UseAllFive/ec2-ssh-stages-fix

Fix for 'ec2:ssh' command when using stages
Merge pull request #38 from UseAllFive/ec2-ssh-key-fix

Make ec2:ssh use the ssh key if configured
Fix issue with .empty? on a symbol.
Updated changelog and version for v1.5.0.pre3, resolves #38, resolves #39 and resolves #40.