Use IUPAC natural abundances in nuc_data.h5

Although #133 updated the isotopic abundances in `abundaces.txt`, that data was not actually getting pulled in to nuc_data.h5. This pull request uses get_isotopic_abundances from pyne.dbgen.isotopic_abundances to create the natural abundance dictionary and uses that in lieu of the KAERI data. There was also a typo in the natural abundance for Ni-58 that I've fixed.

Note that the prebuilt nuc_data.h5 which is used by default does not contain updated values. To generate a nuc_data.h5 with the correct natural abundances, one should run `nuc_data_make --fetch-prebuilt n`. @scopatz Is the prebuilt nuc_data.h5 that is sitting on s3 kept up-to-date somehow?