update gemspec to handle all 3.0.x versions of activerecord
Version bump
Various changes to support rails 3.1 + 3.2

As follows:

1. Bump version to 0.1.5

2. Loosen activerecord dependency from ~> to >=

3. The sqlite3-ruby gem has been renamed to sqlite3

4. Apparently Gemfile.lock shouldn't be commited into gem repos. See:
   http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/

5. Swap deprecated set_table_name for self.table_name=

6. Swap deprecated class_inheritable_accessor for class_attribute

7. Pipeline uses a hack to allow symbols as attribute values by adding a
   quoted_id method to Symbol. But ActiveRecord's quoting has changed
   and it now expects .id for anything with .quoted_id