Arel::SqlLiteral has been moved in rails 42

You will get following error:

```
NameError - uninitialized constant Arel::SqlLiteral:
  activesupport (4.2.0.rc2) lib/active_support/dependencies.rb:533:in
`load_missing_constant'
  activesupport (4.2.0.rc2) lib/active_support/dependencies.rb:184:in
`const_missing'
  geokit-rails (2.0.1) lib/geokit-rails/acts_as_mappable.rb:237:in
`distance_conditions'
  geokit-rails (2.0.1) lib/geokit-rails/acts_as_mappable.rb:108:in
`within'
```
Actually, this has been removed once in arel gems at 5.0 once but add
back, and it looks like they decide to remove it again in 6.0. I think
the safe way is to use SqlLiteral from Arel::Nodes module.
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