Don't give an error when rebuilding documentation

Previously, postgres would give a duplicate key error when a crate was
built more than once:

```
thread 'main' panicked at 'Building documentation failed: Error(Db(DbError { severity: "ERROR", parsed_severity: Some(Error), code: SqlState("23505"), message: "duplicate key value violates unique constraint \"compression_rels_release_algorithm_key\"", detail: Some("Key (release, algorithm)=(1, 0) already exists."), hint: None, position: None, where_: None, schema: Some("public"), table: Some("compression_rels"), column: None, datatype: None, constraint: Some("compression_rels_release_algorithm_key"), file: Some("nbtinsert.c"), line: Some(434), routine: Some("_bt_check_unique") }))', src/bin/cratesfyi.rs:321:21
```

Now, duplicate keys are discarded.

Author: jyn514

src/db/add_package.rs

@@ -361,7 +361,10 @@ fn add_compression_into_database<I>(conn: &Connection, algorithms: I, release_id
 where
     I: Iterator<Item = CompressionAlgorithm>,
 {
-    let sql = "INSERT INTO compression_rels (release, algorithm) VALUES ($1, $2);";
+    let sql = "
+    INSERT INTO compression_rels (release, algorithm)
+    VALUES ($1, $2)
+    ON CONFLICT DO NOTHING;";
     let prepared = conn.prepare_cached(sql)?;
     for alg in algorithms {
         prepared.execute(&[&release_id, &(alg as i32)])?;