Rails 4.1 support

[anowell]

I started with @hale's fix, upgraded rails to 4.1.6, and made a handful of changes to get all the tests passing again. The switch from InstanceTag to Tags::TextField implies this will only work for Rails 4.1 and later.

I also re-factored it a bit (separate commit) to avoid needing to copy parts of the options hash between a JqueryDatePicker::InstanceTag and a TextField. Now you can directly render the JqueryDatePicker::InstanceTag. All tests should still be passing.

[anowell]

I had forgotten to drop ruby 1.9.2 from the travis build. I replaced it with the latest-and-greatest 2.1.4 and now things look happy.

[shireeshj]

is there any change this will be merged into master?