Update vendored zstd c lib to 1.3.4

[elijahandrews]

Tests pass on Go 1.10

[Viq111]

👍 when tests passes.

A great release

[elijahandrews]

Benchmark using mr from silesia corpus (same as #1 and #18):

1.3.4

$ go test -bench . -benchtime 30s
goos: darwin
goarch: amd64
BenchmarkStreamCompression-4 300 131514235 ns/op 75.81 MB/s
BenchmarkStreamDecompression-4 2000 24035926 ns/op 414.82 MB/s
BenchmarkCompression-4 300 128164648 ns/op 77.79 MB/s
BenchmarkDecompression-4 2000 20141948 ns/op 495.01 MB/s
--- BENCH: BenchmarkDecompression-4
zstd_test.go:156: Reduced from 9970564 to 3429982
zstd_test.go:156: Reduced from 9970564 to 3429982
zstd_test.go:156: Reduced from 9970564 to 3429982
PASS
ok _/Users/elijah/src/zstd	202.153s

1.3.3 (current head of branch 1.x)

$ go test -bench . -benchtime 30s
goos: darwin
goarch: amd64
BenchmarkStreamCompression-4 300 130867333 ns/op 76.19 MB/s
BenchmarkStreamDecompression-4 2000 25758836 ns/op 387.07 MB/s
BenchmarkCompression-4 300 122945966 ns/op 81.10 MB/s
BenchmarkDecompression-4 2000 22440310 ns/op 444.31 MB/s
--- BENCH: BenchmarkDecompression-4
zstd_test.go:156: Reduced from 9970564 to 3429982
zstd_test.go:156: Reduced from 9970564 to 3429982
zstd_test.go:156: Reduced from 9970564 to 3429982
PASS
ok _/Users/elijah/src/zstd	208.446s

stream compression: -0.5%
stream decompression: +7%
compression: -4%
decompression: +11%
compressed size: identical