Fix Timex.Duration.parse!/2 was calling parse/1 instead of parse/2

[bfolkens]

Summary of changes

Fixes #795

Currently, Timex.Duration.parse!/2 calls parse/1 instead of parse/2, so it ignores the parser argument. This patch just changes the call to parse/2, in order to preserve the alt parser functionality.

Checklist

• New functions have typespecs, changed functions were updated
• Same for documentation, including moduledocs
• Tests were added or updated to cover changes
• Commits were squashed into a single coherent commit
• Notes added to CHANGELOG file which describe changes at a high-level

NOTE: Timex.Duration.parse isn't currently covered by any of the tests. I'm wondering if that was intentional or I'm missing something, or perhaps they were removed by accident? Happy to write those if needed.