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