Fix Travis docs build script

I changed the conda setup a bit ago to match the recommended way,
but forgot to update the script that builds and pushes the docs
to GitHub Pages.

bin/build_docs.sh

@@ -34,7 +34,7 @@ if [ "$TRAVIS_REPO_SLUG" == "synthicity/pandana" ] && \
         [ "$ACTUAL_TRAVIS_JOB_NUMBER" == "1" ]; then
 
         echo "Installing dependencies"
-        conda install --yes --quiet --prefix $HOME/py sphinx numpydoc
+        conda install --yes --quiet sphinx numpydoc
         pip install sphinx_rtd_theme
 
         echo "Building docs"