ci, travis: fix coveralls installation

.travis.yml

@@ -36,9 +36,6 @@ branches:
   only:
     - master
 
-before_install:
-  - pip install --user cpp-coveralls
-
 script:
   - sh ./build-deps.sh
   - sh ./autogen.sh
@@ -52,6 +49,7 @@ script:
 
 after_success:
   - if [[ "$CC" == "gcc" && "$CODE_COVERAGE" == "ON" ]]; then
+      pip install --user cpp-coveralls
       coveralls
         --build-root .
         --exclude lib