无法在CircleCi 2.0上安装ffmpeg

我有 Ruby on Rails 项目，该项目使用CircleCi来运行测试。过去我使用的是CircleCi 1.0，但现在我迁移到CircleCi 2.0。我在安装ffmpeg时遇到问题。 CircleCi 2.0使用Ubuntu 14.04。我这样安装ffmpeg：

I have Ruby on Rails project which uses CircleCi to run tests. In the past I was using CircleCi 1.0 but now I migrated to CircleCi 2.0. I have problem with installing ffmpeg. CircleCi 2.0 uses Ubuntu 14.04. I install ffmpeg like this:

# ffmpeg installation
sudo add-apt-repository ppa:mc3man/trusty-media -y
sudo apt-get update
sudo apt-get install ffmpeg

我的 circle.yml 配置文件如下：

version: 2
environment:
  TZ: "/usr/share/zoneinfo/America/Los_Angeles"

jobs:
  build:
    parallelism: 2
    working_directory: ~/circleci-survey-builder
    docker:
      - image: circleci/ruby:2.4.1-node
        environment:
          PGHOST: 127.0.0.1
          PGUSER: ubuntu
          RAILS_ENV: test
      - image: circleci/postgres:9.6-alpine
        environment:
          POSTGRES_USER: ubuntu
          POSTGRES_DB: circle_test
          POSTGRES_PASSWORD: ''
    steps:
      - checkout

      - run:
          name: 'Install CircleCI dependencies'
          command: bash deploy/circle-dependencies.sh

      - type: cache-restore
        key: dashboard-{{ checksum "Gemfile.lock" }}

      - run:
          name: 'Install gems'
          command: bundle install --path vendor/bundle

      - type: cache-save
        key: dashboard-{{ checksum "Gemfile.lock" }}
        paths:
          - vendor/bundle

      - run:
          name: 'Install postgresql-client'
          command: sudo apt install postgresql-client

      - run:
          name: 'Create database.yml'
          command: mv config/database.ci.yml config/database.yml

      - run:
          name: Set up SurveyBuilder database
          command: bundle exec rake db:structure:load --trace

      - run:
          name: 'Run tests'
          command: |
            bundle exec rspec spec

在CircleCi上运行构建时返回以下错误：

It returns following error when I run build on CircleCi:

#!/bin/bash -eo pipefail
bash deploy/circle-dependencies.sh

Fetching: bundler-1.16.0.gem (100%)
Successfully installed bundler-1.16.0
1 gem installed
sudo: add-apt-repository: command not found


Ign http://deb.debian.org jessie InRelease

Get:1 http://deb.debian.org jessie-updates InRelease [145 kB]

10% [1 InRelease 13.8 kB/145 kB 10%] [Waiting for headers] [Connecting to securHit http://deb.debian.org jessie Release.gpg

Hit http://deb.debian.org jessie Release

Get:2 http://deb.debian.org jessie-updates/main amd64 Packages [23.2 kB]

Get:3 http://deb.debian.org jessie/main amd64 Packages [9063 kB]

Get:4 http://security.debian.org jessie/updates InRelease [63.1 kB]

Get:5 http://security.debian.org jessie/updates/main amd64 Packages [610 kB]

100% [3 Packages 9063 kB]100% [3 Packages 9063 kB]Fetched 9904 kB in 1s (8178 kB/s)

Reading package lists... 1%
Reading package lists... 61%
Reading package lists... Done


Reading package lists... 1%
Reading package lists... Done


Building dependency tree... 0%
Building dependency tree       


Reading state information... Done

Package ffmpeg is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'ffmpeg' has no installation candidate
Exited with code 100

我该如何解决？