added libmysqlclient-dev as dependency.

`bundle install --without postgres aws` failed for me with:

```
[...]
Installing mysql2 (0.3.11) with native extensions /usr/lib/ruby/1.9.1/rubygems/installer.rb:556:in `rescue in block in build_extensions': ERROR: Failed to buil
d gem native extension. (Gem::Installer::ExtensionBuildError)

        /usr/bin/ruby1.9.1 extconf.rb 
checking for rb_thread_blocking_region()... yes
checking for rb_wait_for_single_fd()... yes
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lm... yes
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lz... yes
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lsocket... no
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lnsl... yes
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lmygcc... no
checking for mysql_query() in -lmysqlclient... no
*** 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
```

Installing libmysqlclient-dev solved the issue for me.