Bump charlock to 0.7.3

This version includes a fix for the encoding lookup table for some encoding aliases in the ICU detection API. The issue was that there were some encodings that could be returned from the detection API which aren't in the list of supported encodings we get from ICU. Because of that the lookup table we build in charlock to find a Ruby-compatible encoding was missing entries for those unlisted encoding aliases.

https://github.com/brianmario/charlock_holmes/pull/69 is the pull which adds them and fixes this issue.