[FIX] .travis.yml,requirements.txt: Move python deps to the right place

Currently, some Python dependencies are being installed manually from
the `.travis.yml` file, using
pip install python_package

Not only that way is a bad practice, but also prevents those Python
modules to be retrieved when an Odoo module is installed via MQT.

In addition, the modules `requests` and `PyPDF2` are removed, as they
already are in Odoo's requirements.

.travis.yml

@@ -35,9 +35,6 @@ install:
   - git clone --depth=1 https://github.com/OCA/maintainer-quality-tools.git ${HOME}/maintainer-quality-tools
   - export PATH=${HOME}/maintainer-quality-tools/travis:${PATH}
   - travis_install_nightly
-  - pip install pycups==1.9.73
-  - pip install PyPDF2==1.18
-  - pip install requests
   - git clone https://github.com/OCA/reporting-engine -b ${VERSION} $HOME/reporting-engine
 
 script:

requirements.txt

@@ -1 +1,2 @@
 zpl2>=1.1
+pycups==1.9.73