only install sphinx requirements when building doc

Author: dbu

.travis.yml

@@ -51,7 +51,7 @@ matrix:
       dist: trusty
 
 install:
-  - pip install -qr Resources/doc/requirements.txt --user
+  - if [[ "$DOCCHECK" = true ]]; then pip install -qr Resources/doc/requirements.txt --user; fi
 
 before_install:
   - sh -c 'if [ "$SYMFONY_VERSION" != "" ]; then composer require --dev --no-update symfony/symfony=$SYMFONY_VERSION; fi;'