Normalise locale string when used by SmartlingTranslator

This adds basic normalisation of the locale string sent to Smartling, to allow for locally specified locales containing underscores, e.g. `xx_YY`. Smartling expects locale strings to contain dashes, e.g. `xx-YY`.

This should be a non-breaking change, existing usage of `xx-yy` locales in `phreepio.json` will continue to work.

This feels like the least-intrusive solution. Looking into adding additional formatting options along the lines of `%locale.xx_YY%` when specifying the destination location felt like it was going to be making too many assumptions about how the locale looked. For example, what if it is xx-yy-zz or some other valid format. As an aside, the RFC for language tags specifies dashes (http://www.ietf.org/rfc/rfc4646.txt), which is also what PHP itself conforms to.