Add gertty-env to .gitignore

If you set up a virtualenv in gertty-env according to the instructions
in the readme, it shows up as an untracked folder.  Add it to the ignore
list.

Change-Id: Ieff29851a8e7a16be53a5a12f14dc2ee9f6e9a5e
Merge "Add gertty-env to .gitignore"