Fix segfault on Darwin

Ever since SQLite in Nixpkgs was updated to 3.8.0.2, Nix has randomly
segfaulted on Darwin:

  http://hydra.nixos.org/build/6175515
  http://hydra.nixos.org/build/6611038

It turns out that this is because the binary cache substituter somehow
ends up loading two versions of SQLite: the one in Nixpkgs and the
other from /usr/lib/libsqlite3.dylib.  It's not exactly clear why the
latter is loaded, but it appears to be because WWW::Curl indirectly loads
/System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation,
which in turn seems to load /usr/lib/libsqlite3.dylib.  This leads to
a segfault when Perl exits:

  #0  0x00000001010375f4 in sqlite3_finalize ()
  #1  0x000000010125806e in sqlite_st_destroy ()
  #2  0x000000010124bc30 in XS_DBD__SQLite__st_DESTROY ()
  #3  0x00000001001c8155 in XS_DBI_dispatch ()
  ...
  #14 0x0000000100023224 in perl_destruct ()
  #15 0x0000000100000d6a in main ()
  ...

The workaround is to explicitly load DBD::SQLite before WWW::Curl.