README: replace /opt/td-agent/embedded/bin/gem -> td-agent-gem

Because td-agent2 has new `td-agent-gem` utility tool.
use V1 Format in README

td-agent2's init script pass `use-v1-config` option.
We can use V1 Format when using td-agent2.
Merge pull request #2 from cosmo0920/fix-README-for-td-agent2

fix readme for td agent2