Fix a misspelled -D name tested in Travis
Fix a misspelled -D name tested in Travis

The code in d2s.c and d2s_table_test.cc refers to
`defined(RYU_ONLY_64_BIT_OPS)`. Previously this macro
was never defined in Travis.

ulfjack/ryu,ulfjack/ryu,ulfjack/ryu,ulfjack/ryu