[travis] Pip upgrade before installing other wheels

.travis.yml

@@ -16,6 +16,7 @@ before_install:
 - sudo add-apt-repository -y ppa:biometrics/bob
 - sudo apt-get update -qq
 - sudo apt-get install --force-yes libboost-all-dev libblitz1-dev libhdf5-serial-dev texlive-latex-recommended texlive-latex-extra texlive-fonts-recommended
+- pip install --upgrade pip
 - pip install --find-links https://www.idiap.ch/software/bob/wheels/travis/ --use-wheel sphinx nose coverage cpp-coveralls
 - pip install --find-links https://www.idiap.ch/software/bob/wheels/travis/ --use-wheel -r requirements.txt --pre coveralls
 install: