Setting gfortran to gcc5. octave was crashing.

Otherwise, using imread() in octave threw:

/nix/store/4fvwfzwg58d7167an550xm1k6m7px443-octave-4.0.0/lib/octave/4.0.0/oct/x86_64-unknown-linux-gnu/__magick_read__.oct: failed to load: /nix/store/w7xr6frwffrl135v7vpxdwmnx8l95j5m-gfortran-4.9.3/lib/libstdc++.so.6: version `GLIBCXX_3.4.21' not found (required by /nix/store/qlxkin1arzwbcpiny6amn8747wp8ndg7-graphicsmagick-1.3.21/lib/libGraphicsMagick++.so.11)

(this is from 16.03, although I push this to staging)