improve error reporting and use rolling timeout

## Changes

1. Uses new `rolling_timeout_exec` npm [package](https://npmjs.org/package/rolling_timeout_exec) which provides `rollingTimeout` options field and will kill child process if no activity is observed on stdout or stderr for `rollingTimeout` milliseconds.

2. Improves error reporting in case of child process error:


### Old:
```sh
$ mrt update
✓ bad-repo
    branch: https://github.com/alanning/xyz.git#master

/Users/alanning/src/js/meteorite/lib/sources/git.js:114
        throw "There was a problem cloning repo: " + self.url +
                                                              ^
There was a problem cloning repo: https://github.com/alanning/xyz.git
Please check https://github.com/oortcloud/meteorite/blob/master/CONTRIBUTING.md#troubleshooting for potential explanations.
```


### New:
```sh
$ mrt update
✓ bad-repo
    branch: https://github.com/alanning/xyz.git#master
ERROR: 128 Command failed: Cloning into '/Users/alanning/.meteorite/source/alanning/xyz'...
remote: Repository not found.
fatal: repository 'https://github.com/alanning/xyz.git/' not found


STDOUT:

STDERR: Cloning into '/Users/alanning/.meteorite/source/alanning/xyz'...
remote: Repository not found.
fatal: repository 'https://github.com/alanning/xyz.git/' not found



/Users/alanning/src/js/meteorite/lib/sources/git.js:124
        throw "There was a problem cloning repo: " + self.url +
                                                              ^
There was a problem cloning repo: https://github.com/alanning/xyz.git
Please check https://github.com/oortcloud/meteorite/blob/master/CONTRIBUTING.md#troubleshooting for potential explanations.
```