Change parameters for configurating Twitter gem.

- config.oauth_token        -> config.access_token
- config.oauth_token_secret -> config.access_token_secret

This commit refs #49.
Install `cert.pem` to /opt/sensu/embedded/ssl/cert.pem.

This commit refs #49.
Merge pull request #49 from kazu634/id/49

[sensu-custom] tw.rbでtwitter gemの仕様変更に対応する