Standardize nextseq symlink ordering

In some cases, glob.glob() returns stuff off the filesystem in a
non-sorted order. This meant that L001_R1_003 didn't necessarily link to
L003_R1_001. This didn't affect anything downstream, but adding sorting
makes it behave more as you'd expect.
Merge pull request #4 from fwip/fix_nextseq_ordering

Standardize nextseq symlink ordering