Use defined path for build trend

Signed-off-by: Michal Čihař <michal@cihar.com>

.travis.yml

@@ -28,7 +28,7 @@ install:
   - pip install -r requirements-optional.txt
   - pip install -r travis/requirements.txt
   - pip install -r travis/requirements-${TRAVIS_DATABASE:-sqlite}.txt
-  - pip install -r $HOME/buildtime-trend/requirements.txt
+  - pip install -r ${BUILD_TREND_HOME}/requirements.txt
 # create databases
 before_script:
   - timestamp.sh before_script