Python 2.6 doesn't provide subprocess.check_output, which we use to

determine whether we can call certain looks (like npm) during package

dependency installation. An alternative is subprocess.check_call, which

works but outputs results to stdout, and isn't as nice as check_output

for our purposes.
This change conditionally chooses which call to make, based on the

version of Python. On 2.7, we use the nicer check_output, whereas on

2.7, we use the less-nice-but-still-suitable check_call.

Installed dependencies on both Python 2.6 and 2.7.