Update to 4.007

Upstream changes:
4.007 Tues Apr 9, 2019
    * Sereal: restore write_sereal(), read_sereal() as aliases to write_sereal_file()
      and read_sereal_file(). Better tests for exported subs. Add SRL_UNCOMPRESSED,
      SRL_SNAPPY, SRL_ZLIB, SRL_ZSTD exports.
    * Decoder: tweak docs for looks_like_sereal/scalar_looks_like_sereal

4.006 Mon Apr 8, 2019
    * Sereal: fix read_sereal_file(),

(wen)