Also cache pip in travis.

.travis.yml

@@ -17,6 +17,7 @@ before_cache:
 cache:
   ccache: true
   directories:
+    - $HOME/.cache/pip
     - $HOME/.gradle/caches/
     - $HOME/.gradle/wrapper/
     - $HOME/.android/build-cache

travis/install_extra_deps.sh

@@ -6,8 +6,8 @@ orig_dir=$(pwd)
 
 sudo apt-get update -qq
 sudo apt-get install -qq python3-pip zlib1g-dev libjpeg-dev
-pip3 install --user meson==0.43.0
-pip3 install --user pillow
+pip3 install --user --upgrade pip wheel
+pip3 install --user pillow meson==0.43.0
 
 # ninja
 git clone git://github.com/ninja-build/ninja.git