Update Snippy v4.6

[gallardoalba]

FOR CONTRIBUTOR:

• - I have read the CONTRIBUTING.md document and this tool is appropriate for the tools-iuc repo.
• - License permits unrestricted use (educational + commercial)
• - This PR adds a new tool or tool collection
• - This PR updates an existing tool or tool collection
• - This PR does something else (explain below)

This PR updates the Snippy tool up to version 4.6. According to the developers, it includes the following improvements:

• Now runs 2 to 3 times faster due to new threading support in markdup and fixmate
• All required Linux binaries are now bundled to simplify container building
• Reinstated TRAVIS testing
• Improved CPU spreading logic for main alignment pipeline stages
• Variants near contig ends will be less likely to be missed due to improved samclip
• Bundled tools now fully python3 compatible