Use regular hash instead of Hashish object in session dump

Without this patch, unit tests will fail intermittently on ruby 1.8.7. The
Hashish initializer calls the .dup method on its argument, and on 1.8.7 key
order is not guaranteed to be preserved. This causes the equality test for the
session to fail sometimes. This patch resolves the problem by using a regular
hash instead of a Hashish object in the session dump method. The dump method
does not need to use a Hashish object since it is immediately turning it into
JSON, which has no need to change symbols to strings and therefore has no need
to use the Hashish object.
Require pathname

Without this patch, passing an invalid path into an Aviator::Session object
raises an uninitialized constant error when it should be raising an
InvalidConfigFilePathError. This patch adds the pathname gem to
lib/aviator/core.rb so that Pathname is accessible from other modules.