gnu: Replace versioned gfortran inputs with just "gfortran".

* gnu/packages/julia.scm (julia)[inputs]: Replace "gfortran-4.8" with
  "gfortran".
* gnu/packages/machine-learning.scm (randomjungle)[native-inputs]: Likewise.
* gnu/packages/maths.scm (arpack-ng, lapack, petsc, superlu)
   (superlu-dist, openblas, openspecfun, atlas)[inputs]: Likewise.
   (octave)[native-inputs]: Likewise.
   (gmsh)[propagated-inputs]: Likewise.
* gnu/packages/mpi.scm (openmpi)[inputs]: Likewise.
* gnu/packages/python.scm (python-numpy-bootstrap)[native-inputs]: Likewise.
* gnu/packages/python.scm (python-scipy)[native-inputs]: Likewise.
* gnu/packages/statistics.scm (r)[inputs]: Replace "gfortran-4.9" with
  "gfortran".