[Fix #1485] Ignore procs in SymbolProc

A Rails-specific fix. See https://github.com/bbatsov/rubocop/issues/1485
for details.
[Fx #1473] Recognize operator []= in MultilineOperationIndentation

The []= operator is a form of assignment and should be
recognized as such in MultilineOperationIndentation where
assignment is handled in a special way.