[ci] Don't cache pip anymore (faster build)

.travis.yml

@@ -4,7 +4,7 @@ language: python
 # the build almost instantly (but adding to the cache takes a while).
 cache:
     directories:
-        - $HOME/.cache/pip
+#        - $HOME/.cache/pip
         - $HOME/virtualenv/python3.6.7/lib/python3.6/site-packages
 env:
     # List packages that you want to reinstall everytime
@@ -21,7 +21,6 @@ install:
     - for package in $NO_CACHE; do pip uninstall -y ${package}; done
     - pip install numpy Cython
     - pip install pytest coverage codecov
-#    - pip install pb_bss@git+https://github.com/mpariente/pb_bss
     - pip install -r requirements.txt
     - pip install -r docs/requirements.txt
     - pip install -e . --no-deps