biology/jellyfish: Update to version 2.2.6

Upstream changes since version 2.2.4:

• Fix compilation warnings/errors with newer version of gcc (5.3 & 6.1).
• Fix bug in parsing function that made it stop on sequences that consist of only empty lines.
• New methods in bindings to enumerate the mers of a string.

Approved by: AMDmi3 (mentor)
Differential Revision: https://reviews.freebsd.org/D8204