SwiftLint2

change VariableNameRule to allow capital static let

There is at least one semi-common case where capitalized names should
probably be allowed: in OptionSetType classes, the standard pattern is
to use `static let = Foo` to declare each option for the type.

Apple docs demostrating this:
https://developer.apple.com/library/prerelease/ios/documentation/Swift/Reference/Swift_OptionSetType_Protocol/index.html

One downside of this implementation is that it would *also* pass a
capitalized "static var Foo", which is probably not desirable. It's not
clear to me how feasible addressing this is: it seems like SourceKitten
might need to be updated to support this better? Using `static var` at
all is code smell, though, so hopefully this isn't a huge deal. Perhaps
there should even be a rule identifying `static var` and considering it
a violation?