If python is your thing (I see ‘conda’ in your directory structure), a very minimal version of ROOT can be installed using pip. Its purpose is to install cling, but Core depends as much on I/O as vice versa, so it still contains full I/O as well:
https://pypi.python.org/pypi/cppyy-cling/
The source for this is created by a script:
that not only removes all extraneous stuff, but also edits the cmake files and some of the source files to kill spurious dependencies. It does add some patches, but those should be mostly harmless (certainly if all you do is I/O).
The PyPA package support wheels and the binary is python-agnostic, so local caching, virtualenv, etc. all play nice.