Explicitly create the virtualenv with Python2 (#823)

README.md

@@ -56,7 +56,7 @@ The easiest way to get an Algo server running is to let it set up a _new_ virtua
 
 4. **Install Algo's remaining dependencies.** Use the same Terminal window as the previous step and run:
     ```bash
-    $ python -m virtualenv env && source env/bin/activate && python -m pip install -U pip && python -m pip install -r requirements.txt
+    $ python -m virtualenv --python=`which python2` env && source env/bin/activate && python -m pip install -U pip && python -m pip install -r requirements.txt
     ```
     On macOS, you may be prompted to install `cc`. You should press accept if so.