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
Merge remote-tracking branch 'github/master'