Long version strings get id "version-long" in download.

"2.11.0-M4" doesn't fit at 20px, but it will at 18px,
and slightly shifted to the left.

Would be cool if we could auto-size this -- for now,
looking at string length...
Merge pull request #87 from adriaanm/button-size

Adapt font size of download button when version is long.