Update to latest Snappy to fix compilation issue on latest MacOS XCode (#6496)

Summary:
* **macOS version:** 10.15.2 (Catalina)
* **XCode/Clang version:** Apple clang version 11.0.0 (clang-1100.0.33.16)

Before this bugfix the error generated is:

```
In file included from ./util/compression.h:23:
./snappy-1.1.7/snappy.h:76:59: error: unknown type name 'string'; did you mean 'std::string'?
  size_t Compress(const char* input, size_t input_length, string* output);
                                                          ^~~~~~
                                                          std::string
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/iosfwd:211:65: note: 'std::string' declared here
typedef basic_string<char, char_traits<char>, allocator<char> > string;
                                                                ^
In file included from db/builder.cc:10:
In file included from ./db/builder.h:12:
In file included from ./db/range_tombstone_fragmenter.h:15:
In file included from ./db/pinned_iterators_manager.h:12:
In file included from ./table/internal_iterator.h:13:
In file included from ./table/format.h:25:
In file included from ./options/cf_options.h:14:
In file included from ./util/compression.h:23:
./snappy-1.1.7/snappy.h:85:19: error: unknown type name 'string'; did you mean 'std::string'?
                  string* uncompressed);
                  ^~~~~~
                  std::string
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/iosfwd:211:65: note: 'std::string' declared here
typedef basic_string<char, char_traits<char>, allocator<char> > string;
                                                                ^
2 errors generated.
make: *** [jls/db/builder.o] Error 1
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6496

Differential Revision: D20389254

Pulled By: pdillinger

fbshipit-source-id: 2864245c8d0dba7b2ab81294241a62f2adf02e20

Makefile

@@ -1808,8 +1808,8 @@ ZLIB_DOWNLOAD_BASE ?= http://zlib.net
 BZIP2_VER ?= 1.0.6
 BZIP2_SHA256 ?= a2848f34fcd5d6cf47def00461fcb528a0484d8edef8208d6d2e2909dc61d9cd
 BZIP2_DOWNLOAD_BASE ?= https://downloads.sourceforge.net/project/bzip2
-SNAPPY_VER ?= 1.1.7
+SNAPPY_VER ?= 1.1.8
-SNAPPY_SHA256 ?= 3dfa02e873ff51a11ee02b9ca391807f0c8ea0529a4924afa645fbf97163f9d4
+SNAPPY_SHA256 ?= 16b677f07832a612b0836178db7f374e414f94657c138e6993cbfc5dcc58651f
 SNAPPY_DOWNLOAD_BASE ?= https://github.com/google/snappy/archive
 LZ4_VER ?= 1.9.2
 LZ4_SHA256 ?= 658ba6191fa44c92280d4aa2c271b0f4fbc0e34d249578dd05e50e76d0e5efcc