Update env def for pip --ignore-installed

Dockerfile_snippet.md

@@ -1,7 +1,7 @@
 Copy and paste the following Dockerfile snippet into the Dockerfile section for a new environment:
 
 ```
-RUN pip install imdbpy wget tmdbsimple tensorflow gensim stop_words
+RUN pip install --ignore-installed imdbpy wget tmdbsimple tensorflow gensim stop_words
 RUN pip install git+https://github.com/dominodatalab/python-domino.git
 RUN echo 'if [ -z ${PYTHONPATH+x} ]; then export PYTHONPATH=${DOMINO_WORKING_DIR}; else export PYTHONPATH=${DOMINO_WORKING_DIR}:${PYTHONPATH}; fi' >> /home/ubuntu/.domino-defaults
 RUN R --no-save -e "install.packages(c('flexdashboard', 'rmarkdown'))"