Bump Flint version to 0.5.1

[noCharger]

Description

Bumped Flint version to 0.5.1. Ref: #360

Testing

$ sbt clean assembly
$ find . -name "*-0.5.0-SNAPSHOT.jar"

$ find . -name "*-0.5.1-SNAPSHOT.jar"

% find . -name "*-0.5.1-SNAPSHOT.jar"
./integ-test/target/scala-2.12/integ-test-assembly-0.5.1-SNAPSHOT.jar
./spark-sql-application/target/scala-2.12/sql-job-assembly-0.5.1-SNAPSHOT.jar
./flint-spark-integration/target/scala-2.12/flint-spark-integration-assembly-0.5.1-SNAPSHOT.jar
./ppl-spark-integration/target/scala-2.12/ppl-spark-integration-assembly-0.5.1-SNAPSHOT.jar
./flint-commons/target/scala-2.12/flint-commons-assembly-0.5.1-SNAPSHOT.jar

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[noCharger]

Please not merge this one until the 0.5 offically released.

[dai-chen]

Please create tag and release for 0.5.0 before merge. Thanks!