fix npm install/Makefile to no longer throw 'fatal' error when trying to get date from git/hg

for example:

``` shell
marcos@smaland in ~/projects/blerg
○ npm install atoll
npm http GET https://registry.npmjs.org/atoll
npm http 200 https://registry.npmjs.org/atoll

> atoll@0.7.3 install /Users/marcos/projects/blerg/node_modules/atoll
> make atoll

fatal: Not a git repository (or any of the parent directories): .git
Building ./dist/atoll.js 0.7.3
atoll@0.7.3 ./node_modules/atoll
```
