Add option to specify an apt cache.
Only set it up when a cache is configured.
Don't use regexp when the command is a string.

This escapes special characters like {} and ' in unfortunate ways.
Add tests for apt cache configuration.
Allow setting a custom npm proxy.
Who put that extra quote sign in here?
Change specs to use raw generated script for apt cache.

Unfortunately evaluating the build script removes the pipes from
the output, and using tee is currently the best approach I can
think of.
Remove todo.
Redirect tee line to /dev/null.
Change config for apt cache.

Now in line ith the npm cache config.
Change hosts to be fetched from data directly.
Merge pull request #132 from travis-ci/mm-apt-cache

Add auto setup of an apt cache to the build process