Prevent Time input incompatibility in Rails 4

In Formastic, framengs in timeish.rb are defined by the concatenation of date_fragments + time_fragments [code timeish.rb](https://github.com/justinfrench/formtastic/blob/master/lib/formtastic/inputs/base/timeish.rb#L112)

Prior to this [commit](https://github.com/justinfrench/formtastic/commit/a22b424f12cb414e286eef03a0f09fb1e77bc4ba#lib/formtastic/inputs/base/timeish.rb), i18n_date_fragments was returning string and not symbols.

In Formastic-bootstrap, to build a datetime, a list of fragments is defined only with symbols [timeish.rb](https://github.com/mjbellantoni/formtastic-bootstrap/blob/master/lib/formtastic-bootstrap/inputs/base/timeish.rb#L37) so fragments for time could not be found.

To avoid this, I converted every fragment of the list into symbols. It will make the trick until Formstatic release a new version with this commit listed above.

Should solve this [issue](https://github.com/mjbellantoni/formtastic-bootstrap/issues/59)