pwrite and pread are unsupported

[voxik]

Testing against Ruby 2.5, I observe following errors:

  1) Failure:
FakeFSTest#test_pwrite_method_in_file_is_in_fake_fs_file [/builddir/build/BUILD/fakefs-0.13.1/usr/share/gems/gems/fakefs-0.13.1/test/fakefs_test.rb:1409]:
pwrite method is not available in File :(

  2) Failure:
FakeFSTest#test_pread_method_in_file_is_in_fake_fs_file [/builddir/build/BUILD/fakefs-0.13.1/usr/share/gems/gems/fakefs-0.13.1/test/fakefs_test.rb:1409]:
pread method is not available in File :(

These are going to be triggered if #389 is merged

[grosser]

can you add them in a PR ?
... also would be nice to change wording from pread method is not available in File :( -> please add pread method to File! :)