Loosen dependency so newer, working sycks can be used

[technicalpickles]

Without this, installation fails with:

In file included from ./syck.h:20:
/Users/josh.nichols/.rubies/ruby-2.7.4/include/ruby-2.7.0/ruby/st.h:186:52: error: unknown type name 'VALUE'
void rb_hash_bulk_insert_into_st_table(long, const VALUE *, VALUE);
                                                   ^
/Users/josh.nichols/.rubies/ruby-2.7.4/include/ruby-2.7.0/ruby/st.h:186:61: warning: type specifier missing,
defaults to 'int' [-Wimplicit-int]
void rb_hash_bulk_insert_into_st_table(long, const VALUE *, VALUE);
                                                            ^
1 warning and 1 error generated.
make: *** [gram.o] Error 1

make failed, exit code 2

Newer versions of syck are able to compile and install, but there's other issues to using.

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