TL;DR

libpcap-dev をインストールすれば良い。

dependencies:
  pre:
    - sudo apt-get install libpcap-dev

背景

bundle install が以下のようなエラーでコケるようになってしまった。
Ubuntu 12.04 から Ubuntu 14.04 に切り替えたタイミングでエラーが起きるようになった。

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

current directory:
/home/ubuntu/dns/vendor/bundle/ruby/2.4.0/gems/pcaprub-0.12.4/ext/pcaprub_c
/opt/circleci/ruby/ruby-2.4.1/bin/ruby -r ./siteconf20170413-11999-vviegb.rb
extconf.rb

[*] Running checks for pcaprub_c code...
platform is x86_64-linux
checking for ruby/thread.h... yes
checking for rb_thread_blocking_region()... no
checking for rb_thread_call_without_gvl()... yes
checking for pcap_open_live() in -lpcap... no
checking for pcap_setnonblock() in -lpcap... no
creating Makefile

current directory:
/home/ubuntu/dns/vendor/bundle/ruby/2.4.0/gems/pcaprub-0.12.4/ext/pcaprub_c
make "DESTDIR=" clean

current directory:
/home/ubuntu/dns/vendor/bundle/ruby/2.4.0/gems/pcaprub-0.12.4/ext/pcaprub_c
make "DESTDIR="
compiling pcaprub.c
pcaprub.c:11:18: fatal error: pcap.h: No such file or directory
 #include <pcap.h>
                  ^
compilation terminated.
make: *** [pcaprub.o] Error 1

make failed, exit code 2

Gem files will remain installed in
/home/ubuntu/dns/vendor/bundle/ruby/2.4.0/gems/pcaprub-0.12.4 for inspection.
Results logged to
/home/ubuntu/dns/vendor/bundle/ruby/2.4.0/extensions/x86_64-linux/2.4.0-static/pcaprub-0.12.4/gem_make.out

An error occurred while installing pcaprub (0.12.4), and Bundler cannot
continue.
Make sure that `gem install pcaprub -v '0.12.4'` succeeds before bundling.

bundle check --path=vendor/bundle || bundle install --path=vendor/bundle --jobs=4 --retry=3  returned exit code 5

Action failed: bundle install

調べたところ libpcap をインストールせよとのことだった。
https://github.com/pcaprub/pcaprub/issues/49