Relax yajl-ruby gem version

yajl-ruby 1.2.0 is popular version so td command should allow this version.
It causes following warning in td-agent2:

```sh
WARN: Unresolved specs during Gem::Specification.reset:
      yajl-ruby (~> 1.0)
WARN: Clearing out unresolved specs.
Please report a bug if this causes problems.
```