fix(coverage): depend on a modern version of istanbul

0.1.22 has function coverage issues fixed in 0.1.40; use fuzzy version matching
0.1.22: http://bl.ocks.org/johan/raw/5942388/11133099070d95c2f2d36e9f3570881888a7af1e/given.js.html
0.1.40: http://bl.ocks.org/johan/raw/5942446/bb1ce82c73451b7cda711cd60d6984748c15d6c1/given.js.html