env: Ensure proper detection of CircleCI

This commit ensures all parameters required by the python [1] and bash [2] uploader can easily be passed to a docker container [3]

[1] https://github.com/codecov/codecov-python/blob/7386c3c13fc71f237611c780646fa1b44a07dc64/codecov/__init__.py#L337
[2] https://github.com/codecov/codecov-bash/blob/441e2018ed76f4d8cb92dafa732ee24ebebb41f9/codecov#L429
[3] http://docs.codecov.io/docs/testing-with-docker#section-codecov-inside-docker

env

@@ -61,6 +61,7 @@ then
 
 elif [ "$CI" = "true" ] && [ "$CIRCLECI" = "true" ];
 then
+  add "CI"
   add "CIRCLECI"
   add "CIRCLE_BRANCH"
   add "CIRCLE_BUILD_NUM"