Lock yeoman-environment to v1.2.6
Lock yeoman-environment to v1.2.6

They seem to have made a mistake with v1.2.7, since they published no
source code!

Signed-off-by: Juan Cruz Viotti <8493db2a4f45d66df375afe12f7eba59fe1c8d78@gmail.com>

resin-io/nplugm