Set maxRows default in SharkContext.sql to Int.MaxValue

SharkContext.sql gained a new maxRows parameter [here](https://github.com/clearstorydata/shark/commit/009db1aa7cd83fddef4dedb1f992b0c7d27b20bb#src/main/scala/shark/SharkContext.scala), but with a default value that we are not expecting.  This PR changes the default back to what we expect, but it is worth considering whether we shouldn't keep the maxRows=1000 default and add a second parameter of Int.MaxValue at our call site(s).