Resolve conflicts in Apache Spark with Parquet format

gistfile1.txt

Conflict between when adding `parquet-format` to the `sbt` dependencies

```
java.lang.RuntimeException: deduplicate: different file contents found in the following:
path-to-cache/org.slf4j/slf4j-api/jars/slf4j-api-1.7.9.jar:META-INF/maven/org.slf4j/slf4j-api/pom.properties
path-to-cache/com.twitter/parquet-format/jars/parquet-format-2.2.0-rc1.jar:META-INF/maven/org.slf4j/slf4j-api/pom.properties
```

To resolve just exclude the `slf4j-api/pom.properties` from `com.twitter/parquet-format` package:

```
mergeStrategy in assembly <<= (mergeStrategy in assembly) { (old) =>
  {
    case PathList("META-INF", "maven","org.slf4j","slf4j-api", ps) if ps.startsWith("pom") => MergeStrategy.discard
    case x => old(x)
  }
}
```