[travis] Fix push permissions for gh-pages

.travis.yml

+script: ./.travis/build
+after_success: 
+- coveralls -e _travis/install -e tests
+- git config --global user.name "Travis CI"
+- git config --global user.email "thomas.moulard+travis@gmail.com"
+- git remote set-url origin https://thomas-moulard:${GH_TOKEN}@github.com/stack-of-tasks/dynamic-graph.git
+- git fetch origin gh-pages:gh-pages
+- cd _travis/build/doc && ../../../cmake/github/update-doxygen-doc.sh
+before_install: 
+- git submodule update --init --recursive
+- sudo apt-get update -qq
+- sudo apt-get install -qq doxygen doxygen-latex libboost-all-dev libeigen3-dev liblapack-dev libblas-dev gfortran
+- sudo pip install cpp-coveralls --use-mirrors
+compiler: 
+- clang
+- gcc
+env: 
+  global: 
+    secure: VZsHMEVQT5ugtPedZE8bbqEEvnN9KqatLzfmlWIZ9rH0twzDw1xsNMlyIsBZzCe7VeiyUZQinCLWC3eWRSElk8ePqL9ZFl+Ddc17ngPjXqcf3YK9u3XJZvHqFPoLEPO9bh5nlBT+iMFdlqN3GG8SJMUzUuzu07X3CZy5e4RsqQo=
+notifications: 
+  email: 
+  - hpp-source@laas.fr
+branches: 
+  only: 
+  - master
 language: cpp
-compiler:
-  - clang
-  - gcc
-script: "./.travis/build"
-before_install:
-  - git submodule update --init --recursive
-  - sudo apt-get update -qq
-  - sudo apt-get install -qq doxygen doxygen-latex libboost-all-dev libeigen3-dev liblapack-dev libblas-dev gfortran
-  - sudo pip install cpp-coveralls --use-mirrors
-after_success:
-  - coveralls -e _travis/install -e tests
-  - git config --global user.name "Travis CI"
-  - git config --global user.email "thomas.moulard+travis@gmail.com"
-  - git remote set-url origin https://thomas-moulard:${GH_TOKEN}@github.com/stack-of-tasks/dynamic-graph.git
-  - git fetch origin gh-pages:gh-pages
-  - cd _travis/build/doc && ../../../cmake/github/update-doxygen-doc.sh
-notifications:
-  email:
-    - hpp-source@laas.fr
-branches:
-  only:
-    - master
-matrix:
-  allow_failures:
-    - compiler: clang
-env:
-  global:
-    secure: "Slm+HqRT+Vy0SG7E9gEShuHvNz6XWRUmBhVBLVJEaYy/0Lo2qp4OjFEWDjiJwFIiXstW2Wclds4SuewAn3Tc4x7dsY+5uaMCX7S/HPzsBCjLXUwjKasUqEGaVTIYmtFJm57hOkz6K2SLUUV4intkMiwtCiq/R0xe92QTTKkHyfo="
+matrix: 
+  allow_failures: 
+  - compiler: clang