Sandbox autoconf213 to prevent conflicts

Fixes a race condition where autoconf 2.13 may be installed prior to autoconf 2.69+
and end up linked into the main homebrew folder causing homebrew to throw errors
when installing the later non sandboxed version
Set pecl to be the default on darwin
Merge pull request #36 from boxen/fix-sandboxing

Fix autoconf 2.13 sandboxing
[BUGFIX] pm.max_children too small for some (XHR-heavy) projects
Merge pull request #47 from castiron/gb/max-children

[BUGFIX] pm.max_children too small for some (XHR-heavy) projects
Merge pull request #45 from boxen/fixproviders

Set PECL to be the default extension provider
Use autoconfig213 from module resources
Added ssh2 extension
Merge pull request #49 from webflo/patch-2

Use autoconfig213 from module resources
Merge pull request #51 from typhonius/master

Added ssh2 extension
Remove autoconf213 sandboxing which breaks new installs
Merge pull request #52 from boxen/fixautoconf

Remove autoconf213 sandboxing which breaks new installs
Merge pull request #1 from boxen/master

Grab latest from official boxen repo.