Replaced .symbolize with .to_sym in arel nodes.

Without this patch the arel node classes ContainedWithinEquals and
ContainsEquals used .symbolize instead of .to_sym to crate the
operators. This causes a NoMethodError with: undefined method
`symbolize' for "<<=":String.
This patch addresses this issue by using to_sym instead of
symbolize.
Merge pull request #70 from omcnet/fix-arel-node-operators

Replaced .symbolize with .to_sym in arel nodes.