BLD: Change Fortran version flag and string check

The version check flag for GnuFCompiler and Gnu95FCompiler were changed from
`--version` to `-dumpversion`. This simplifies the gnu_version_match code for
gfortran, and makes it possible to drop much of the check code for g77 as
well. This fix addresses issue #5315 and #5321.
Might be easier to understand if written `([0-9.-]+)`, which is documented. I don't lean much one way or the other as to keeping the `-`, but since it never seems to be part of the matched string, omitting it might keep the code just a bit simpler.