Currently, if a version is not provided to the following `pkg install`
command

`../ks pkg install incubator/redis`

, the following error will occur:

`ERROR GET
https://api.github.com/repos/ksonnet/parts/commits/incubator/redis

: 404
Not Found []`

This commit will pull in the latest package from master where the
version is not specified.

Also set the default repository ref to 'master' instead of 'test-reg'.

Signed-off-by: Jessica Yuen <im.jessicayuen@gmail.com>