Only look for previous manifest for nodejs buildpack.
Only look for previous manifest for nodejs buildpack.

Only nodejs provides an actual previous manifest, since only nodejs actually need to look at the prior release's versions of node.
For ruby and dotnet, do not require a prevous manifest.

[#157065173]

Signed-off-by: Tyler Phelan <5713437ebd3bcae14564c2baaa7816b900ed174c@pivotal.io>

cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci