Move Shell into Bosh::Core so Bosh::Stemcell can eventually use it

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>
Promote AvoidPerlisms since $? and $CHILD_STATUS are not the same

Signed-off-by: Colin Humphreys <chumphreys@pivotallabs.com>
gem.rake assumes a version.rb, but gemspecs use BOSH_VERSION!

Signed-off-by: Colin Humphreys <chumphreys@pivotallabs.com>