rails bundle install  pg エラーの対処

##環境

Rails 4.2.2
Bundler version 1.12.5
mac osx el capitan
バージョン 10.11.5

##エラー
bundle install すると

```
Building native extensions.  This could take a while...
ERROR:  Error installing pg:
	ERROR: Failed to build gem native extension.

    current directory: /usr/local/lib/ruby/gems/2.3.0/gems/pg-0.17.1/ext
/usr/local/opt/ruby/bin/ruby -r ./siteconf20160806-45612-1t8l959.rb extconf.rb
checking for pg_config... no
No pg_config... trying anyway. If building fails, please try again with
 --with-pg-config=/path/to/pg_config
checking for libpq-fe.h... no
Can't find the 'libpq-fe.h header
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
	--with-opt-dir
	--with-opt-include
	--without-opt-include=${opt-dir}/include
	--with-opt-lib
	--without-opt-lib=${opt-dir}/lib
	--with-make-prog
	--without-make-prog
	--srcdir=.
	--curdir
	--ruby=/usr/local/Cellar/ruby/2.3.1/bin/$(RUBY_BASE_NAME)
	--with-pg
	--without-pg
	--with-pg-config
	--without-pg-config
	--with-pg_config
	--without-pg_config
	--with-pg-dir
	--without-pg-dir
	--with-pg-include
	--without-pg-include=${pg-dir}/include
	--with-pg-lib
	--without-pg-lib=${pg-dir}/lib

To see why this extension failed to compile, please check the mkmf.log which can be found here:

  /usr/local/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-15/2.3.0/pg-0.17.1/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /usr/local/lib/ruby/gems/2.3.0/gems/pg-0.17.1 for inspection.
Results logged to /usr/local/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-15/2.3.0/pg-0.17.1/gem_make.out
MyMacBookAir:sample_app TanoueSatoshi$ bundle install --without produciton
Fetching gem metadata from https://rubygems.org/
Fetching version metadata from https://rubygems.org/
Fetching dependency metadata from https://rubygems.org/
Resolving dependencies....
Using rake 11.2.2
Using i18n 0.7.0
Using json 1.8.3
Using minitest 5.9.0
Using thread_safe 0.3.5
Using builder 3.2.2
Using erubis 2.7.0
Using mini_portile2 2.1.0
Using pkg-config 1.1.7
Using rack 1.6.4
Using mime-types-data 3.2016.0521
Using arel 6.0.3
Using ansi 1.5.0
Using debug_inspector 0.0.2
Using bundler 1.12.5
Using columnize 0.9.0
Using debugger-linecache 1.2.0
Using slop 3.6.0
Using coderay 1.1.1
Using coffee-script-source 1.10.0
Using execjs 2.7.0
Using thor 0.19.1
Using concurrent-ruby 1.0.2
Using ffi 1.9.14
Using formatador 0.2.5
Using rb-fsevent 0.9.7
Using ruby_dep 1.4.0
Using lumberjack 1.0.10
Using nenv 0.3.0
Using shellany 0.0.1
Using method_source 0.8.2
Using multi_json 1.12.1
Using ruby-progressbar 1.8.1
Installing pg 0.17.1 with native extensions

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

    current directory: /usr/local/lib/ruby/gems/2.3.0/gems/pg-0.17.1/ext
/usr/local/opt/ruby/bin/ruby -r ./siteconf20160806-45880-1vx0pp0.rb extconf.rb
checking for pg_config... no
No pg_config... trying anyway. If building fails, please try again with
 --with-pg-config=/path/to/pg_config
checking for libpq-fe.h... no
Can't find the 'libpq-fe.h header
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
	--with-opt-dir
	--with-opt-include
	--without-opt-include=${opt-dir}/include
	--with-opt-lib
	--without-opt-lib=${opt-dir}/lib
	--with-make-prog
	--without-make-prog
	--srcdir=.
	--curdir
	--ruby=/usr/local/Cellar/ruby/2.3.1/bin/$(RUBY_BASE_NAME)
	--with-pg
	--without-pg
	--with-pg-config
	--without-pg-config
	--with-pg_config
	--without-pg_config
	--with-pg-dir
	--without-pg-dir
	--with-pg-include
	--without-pg-include=${pg-dir}/include
	--with-pg-lib
	--without-pg-lib=${pg-dir}/lib

To see why this extension failed to compile, please check the mkmf.log which can be found here:

  /usr/local/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-15/2.3.0/pg-0.17.1/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /usr/local/lib/ruby/gems/2.3.0/gems/pg-0.17.1 for inspection.
Results logged to /usr/local/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-15/2.3.0/pg-0.17.1/gem_make.out
```

##対処法
```sh
brew install postgresql
```
で対処できました．