Replace deprecated CircleCI convenience images (#513)

https://discuss.circleci.com/t/legacy-convenience-image-deprecation/41034

.circleci/config.yml

@@ -13,11 +13,11 @@ executors:
   python-executor:
     working_directory: ~/pantheon-systems/fusedav
     docker:
-      - image: circleci/python:2.7.15
+      - image: cimg/python:2.7.18
   ruby-executor:
     working_directory: ~/pantheon-systems/fusedav
     docker:
-      - image: circleci/ruby:2.4
+      - image: cimg/ruby:2.4.10
 
 jobs:
   build-test:
@@ -28,7 +28,7 @@ jobs:
       - setup_remote_docker
       - run:
           name: Install cpplint
-          command: sudo pip install cpplint
+          command: pip install cpplint
       - run:
           name: Hack GitHub credentials into git URL
           command: |
@@ -40,7 +40,7 @@ jobs:
          name: Call cpplint; expected to fail, so set things up to pass
          command: |
            set +eo pipefail
-           /usr/local/bin/cpplint --filter=-whitespace/line_length src/*
+           cpplint --filter=-whitespace/line_length src/*
            true
       - run:
          name: Build the image