Upgrade charlock_holmes to v0.7.7

This fixes some build issues on MacOS: https://github.com/brianmario/charlock_holmes/compare/0.7.6..v0.7.7

I was having issues with v0.7.6:

have_header: checking for unicode/ucnv.h... -------------------- no

"clang -E -I/Users/stanhu/.rbenv/versions/2.6.3/include/ruby-2.6.0/x86_64-darwin18 -I/Users/stanhu/.rbenv/versions/2.6.3/include/ruby-2.6.0/ruby/backward -I/Users/stanhu/.rbenv/versions/2.6.3/include/ruby-2.6.0 -I. -I/Users/stanhu/.rbenv/versions/2.6.3/include -I/usr/local/opt/openssl/include -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -D_DARWIN_UNLIMITED_SELECT -D_REENTRANT    -O3 -Wno-error=shorten-64-to-32  -pipe  conftest.c -o conftest.i"
conftest.c:3:10: fatal error: 'unicode/ucnv.h' file not found
#include <unicode/ucnv.h>
         ^~~~~~~~~~~~~~~~
1 error generated.
checked program was:
/* begin */
1: #include "ruby.h"
2:
3: #include <unicode/ucnv.h>
/* end */

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