Split up travis requirements

We don't need to install database backends in all cases.
Signed-off-by: Michal Čihař <michal@cihar.com>

.travis.yml

@@ -19,7 +19,8 @@ install:
   - pip install Django==$DJANGO_VERSION
   - pip install -r requirements.txt
   - pip install -r requirements-optional.txt
-  - pip install -r requirements-travis.txt
+  - pip install -r travis/requirements.txt
+  - pip install -r travis/requirements-$TRAVIS_DATABASE.txt
 # generate configuration and create databases
 before_script:
   - mysql -e 'create database weblate;'

travis/requirements-mysql.txt

+mysql-python

travis/requirements-postgresql.txt

+psycopg2

travis/requirements-sqlite.txt

+

requirements-travis.txt → travis/requirements.txt

 pep8
 coveralls
 pylint
-mysql-python
-psycopg2
 selenium
 httpretty