Let pip install multiple packages at once

Pip can install multiple dependencies at the same time, so there is no need to
use xargs -n1. It's significantly slower with -n1, so let's not do it with no
reason.

README.rst

@@ -427,7 +427,7 @@ to install using pip and a virtualenv::
 
     virtualenv env
     source env/bin/activate
-    python synapse/python_dependencies.py | xargs -n1 pip install
+    python synapse/python_dependencies.py | xargs pip install
     pip install setuptools_trial mock
 
 This will run a process of downloading and installing all the needed
@@ -650,4 +650,3 @@ matrix.org on.  The default setting is currently 0.1, which is probably
 around a ~700MB footprint.  You can dial it down further to 0.02 if
 desired, which targets roughly ~512MB.  Conversely you can dial it up if
 you need performance for lots of users and have a box with a lot of RAM.
-

jenkins/prepare_synapse.sh

@@ -15,6 +15,6 @@ tox -e py27 --notest -v
 
 TOX_BIN=$TOX_DIR/py27/bin
 $TOX_BIN/pip install setuptools
-python synapse/python_dependencies.py | xargs -n1 $TOX_BIN/pip install
-$TOX_BIN/pip install lxml
-$TOX_BIN/pip install psycopg2
+{ python synapse/python_dependencies.py
+  echo lxml psycopg2
+} | xargs $TOX_BIN/pip install