Depend on a better istanbul version

0.1.22 has function coverage issues: http://bl.ocks.org/johan/raw/5942388/11133099070d95c2f2d36e9f3570881888a7af1e/given.js.html
0.1.40 doesn't: http://bl.ocks.org/johan/raw/5942446/bb1ce82c73451b7cda711cd60d6984748c15d6c1/given.js.html