Fail to compile sibyll_dgenerator.f with gcc 4.8.1

[DavidWalz]

A current version of gfortran refuses to compile sibyll.
The following diff makes it compile.

6588a6589
>       IMPLICIT DOUBLE PRECISION (A-H,O-Z)
6608a6610
>       IMPLICIT DOUBLE PRECISION (A-H,O-Z)

[PeterSchiffer]

This problem exists for a while now. Even with older gcc versions.
In the README we recommend to downgrade the gfortran version to 4.4
It is just a workaround, but unless a fortran expert shows up to fix it, I don't think it is worth the effort.