fix: erpnext image build

improve travis.py
python packages not installed in editable mode
use venv module instead of virtualenv

build/erpnext-worker/Dockerfile

@@ -1,5 +1,5 @@
 ARG GIT_BRANCH=develop
 FROM frappe/frappe-worker:${GIT_BRANCH}
 
-ARG GIT_BRANCH=develop
+ARG GIT_BRANCH
 RUN install_app erpnext https://github.com/frappe/erpnext ${GIT_BRANCH}