Fix: Agnoster theme added venv name even if disabled
See http://www.virtualenv.org/en/latest/index.html\?highlight\=virtual_env_disable_prompt\#activate-script for info regarding this.
Merge pull request #2184 from jmagnusson/agnoster_respect_venv_disable_prompt

Fix: Agnoster theme doesn't respect VIRTUAL_ENV_DISABLE_PROMPT