work around noisy 'conflicting chdir during another chdir block' warnings

If the popen method is used with the :chdir option within a Dir.chdir
block on ruby 1.9 (fex inside a in_build_dir block), a warning is issued that is needless distraction here
since we're about to exec anyway.

See the implementation of Dir.chdir
http://www.ruby-doc.org/core-1.9.3/Dir.html#method-c-chdir
Merge pull request #303 from santouras/master

Adding PPA's fails if not being run as root
Use PathHelpers.cd to wrap the chdir logic and avoid two exec calls.
Merge branch 'chdir-warning'
`writable?` -> `writable_real?`

Fixes subtle issues with writable detection, see:
https://github.com/Homebrew/homebrew/commit/8aa4f9dd3bbd885350776171d3d9
Updated Debian 7.4 (wheezy) has been released, see http://www.debian.org/News/2014/20140208

Signed-off-by: Laurent Vallar <val@zbla.net>
Merge pull request #305 from Val/add_7.4_wheezy_debian_version

Add Debian 7.4 / wheezy codename support.
Merge pull request #304 from tamird/master

`writable?` -> `writable_real?`
v0.17.7 - such PRs
wowe

From @tamird:
- Use File.writable_real? to avoid a questionable ruby stdlib group write access check

From @schnittchen:
- Work around a superflous ruby warning when Dir.chdir is used

From @bearded:
- Fix boolean defaults passed to accepts_*_for declarations
- Remove unused String#colorize! start_at arg
- Fix the rubinius entries in .travis.yml

From @santouras:
- Run via sudo when required when adding apt ppas.

From @Val:
- Add a version declaration for debian-7.3, and then debian-7.4
  (This shouldn't be necessary, I'll refactor it someday)
Update NPM installation script URL
Follow npmjs.org redirects in case this moves again.
Merge branch 'npm-url'
v0.17.8 - A little update to fix the URL in 'npm.src'.

Thanks to Timothy Reen for this patch (and for noticing, since I don't
use npm).