Remove 2.4.x support

[chris-twiner]

• Migrate the getPushdowns to a fixed function using relfection per: https://github.com/chris-twiner/frameless/blob/temp/343_LitRule/dataset/src/test/scala/frameless/optimiser/LitTests.scala#L63
• Revaluate for other 3.x functions that can be pulled out of version specifics
• Move all function _ into just "function"
• Remove manual Ordering in comparableMaps impl
• Remove UnresolvedNamedLambdaVariableT
• Use scalaCompatVersion for jarjar abrams (as of 1.3 it's 2.12 as no 2.11 exists)

[chris-twiner]

UnresolvedNamedLambdaVariableT is in shim, getPushDowns in testing. the rest is done in 69ec16c