Tweak logic for writing prefix files to a cache in --build-only mode.
Tweak logic for writing prefix files to a cache in --build-only mode.

Just write a newline separated text file containing all the file names instead
of a json file.  We don't need json here, and json.dumps() doesn't even work
on 2.x in this instance -- it was raising a unicode/str error.

frol/conda-build,mwcraig/conda-build,sandhujasmine/conda-build,shastings517/conda-build,sandhujasmine/conda-build,ilastik/conda-build,mwcraig/conda-build,rmcgibbo/conda-build,takluyver/conda-build,ilastik/conda-build,rmcgibbo/conda-build,shastings517/conda-build,shastings517/conda-build,frol/conda-build,ilastik/conda-build,takluyver/conda-build,takluyver/conda-build,frol/conda-build,dan-blanchard/conda-build,sandhujasmine/conda-build,dan-blanchard/conda-build,dan-blanchard/conda-build,rmcgibbo/conda-build,mwcraig/conda-build