.travis.yml: Install gfortran

Author: Matthias Koeppe

.travis.yml

@@ -27,3 +27,11 @@ after_success:
 before_script:
 - openssl aes-256-cbc -K $encrypted_f96cc3b3888c_key -iv $encrypted_f96cc3b3888c_iv
   -in .travis_ci_gh_pages_deploy_key.enc -out .travis_ci_gh_pages_deploy_key -d || true
+
+addons:
+  apt:
+    packages:
+     # On 8.0 with trusty, Fortran compiler is needed.
+     # Otherwise, numpy cannot be imported.
+     # Also, optional package installs in .travis-install.sh will trigger build of sage gcc.
+     - gfortran