Blob is not supported in PostgreSQL

Since Blob is not supported by PostgreSQL as spotted by https://github.com/src-d/code-annotation/pull/206#pullrequestreview-102665598 its support was dropped by that https://github.com/src-d/code-annotation/pull/206

We should restore the PostgreSQL support and then revert https://github.com/src-d/code-annotation/pull/206/commits/94d41ab24c655a5f4b85c6960de882fef6ea730e if needed