Format year of monthly tick lables with 2 digits instead of 4.
Format year of monthly tick lables with 2 digits instead of 4.

Since commit 7b2dfd0 (Oct 15, 2013) the format of monthly tick
labels is `%b %Y` (e.g. Oct 2013). Before the format was '%b %y'
(e.g. Oct 13). Revert to the later format because it seems to be
an accidental change.

  * `dygraph.js`:
    Revert format of monthly tick labels from `%b %Y` to `%b %y`.

reinert/dygraphs,jmptrader/dygraphs,klausw/dygraphs,Akiyah/dygraphs,Akiyah/dygraphs,pshevtsov/dygraphs,danvk/dygraphs,reinert/dygraphs,Yong-Lee/dygraphs,petechap/dygraphs,mcanthony/dygraphs,mantyr/dygraphs,timeu/dygraphs,kbaggott/dygraphs,Yong-Lee/dygraphs,Yong-Lee/dygraphs,pshevtsov/dygraphs,reinert/dygraphs,danvk/dygraphs,witsa/dygraphs,vhotspur/dygraphs,klausw/dygraphs,mcanthony/dygraphs,mariolll/dygraphs,Yong-Lee/dygraphs,jmptrader/dygraphs,timeu/dygraphs,danvk/dygraphs,kbaggott/dygraphs,klausw/dygraphs,timeu/dygraphs,reinert/dygraphs,danvk/dygraphs,pshevtsov/dygraphs,Yong-Lee/dygraphs,mariolll/dygraphs,kbaggott/dygraphs,petechap/dygraphs,witsa/dygraphs,reinert/dygraphs,vhotspur/dygraphs,kbaggott/dygraphs,Akiyah/dygraphs,davidmsibley/dygraphs,petechap/dygraphs,mariolll/dygraphs,jmptrader/dygraphs,mantyr/dygraphs,pshevtsov/dygraphs,witsa/dygraphs,mantyr/dygraphs,witsa/dygraphs,davidmsibley/dygraphs,vhotspur/dygraphs,witsa/dygraphs,klausw/dygraphs,davidmsibley/dygraphs,mantyr/dygraphs,petechap/dygraphs,mcanthony/dygraphs,Akiyah/dygraphs,timeu/dygraphs,mariolll/dygraphs,jmptrader/dygraphs,mantyr/dygraphs,mariolll/dygraphs,mcanthony/dygraphs,vhotspur/dygraphs,davidmsibley/dygraphs,davidmsibley/dygraphs,pshevtsov/dygraphs,klausw/dygraphs,kbaggott/dygraphs,vhotspur/dygraphs,petechap/dygraphs,mcanthony/dygraphs,Akiyah/dygraphs,jmptrader/dygraphs,danvk/dygraphs,timeu/dygraphs