Small change in	handling of slideIndices key to	account for float conversion

Older manifests	like Solar had slideIndices keys like 0.0, which
breaks the slide section function because parseFloat converts it
to 0, and then slideIndices[0] would not be found.  This fixes it
by using the actual key to dereference.
Merge pull request #2101 from Stanford-Online/jbau-slide-selection-fix

Small change in	handling of slideIndices key to account for float conversion