SPARK-1483: Rename minSplits to minPartitions in public APIs

https://issues.apache.org/jira/browse/SPARK-1483

From the original JIRA: " The parameter name is part of the public API in Scala and Python, since you can pass named parameters to a method, so we should name it to this more descriptive term. Everywhere else we refer to "splits" as partitions." - @mateiz