Use travis_retry for installing dependencies on Travis CI

.travis.yml

@@ -10,10 +10,10 @@ env:
 before_script:
   - if [[ $DATABASE_URI == postgresql* ]]; then psql -c 'create database travis_ci_test;' -U postgres; fi
 install:
-  - pip install -r requirements.txt
-  - pip install -r dev-requirements.txt
-  - if [[ $DATABASE_URI == postgresql* ]]; then pip install psycopg2; fi
-  - pip install coveralls
+  - travis_retry pip install -r requirements.txt
+  - travis_retry pip install -r dev-requirements.txt
+  - if [[ $DATABASE_URI == postgresql* ]]; then travis_retry pip install psycopg2; fi
+  - travis_retry pip install coveralls
 script:
   - coverage run --source=flask_dance runtests.py
 after_success: