BLD: Cache pip downloads on Travis

This enables the cache feature of Travis container-based ingrastructure.
Pip's download/build cache is preserved between jobs, speeding up the
install phase.
Installation of nose etc is moved to after pip has been upgraded so that
wheels are favoured.
Sadly this has no effect on Cython - it's still built from source every time.

.travis.yml

@@ -16,6 +16,10 @@ addons:
       # Speedup builds, particularly when USE_CHROOT=1
       - eatmydata
 
+cache:
+  directories:
+    - $HOME/.cache/pip
+
 python:
   - 2.6
   - 2.7
@@ -60,11 +64,11 @@ before_install:
   # We therefore control our own environment, avoid travis' numpy
   - virtualenv --python=python venv
   - source venv/bin/activate
-  - pip install nose
-  # pip install coverage
   - python -V
   - pip install --upgrade pip setuptools
   # Speed up install by not compiling Cython
+  - pip install nose
+  # pip install coverage
   - pip install --install-option="--no-cython-compile" Cython
   - popd