Omnibus software updates

Updated omnibus-software to the latest master https://github.com/chef/omnibus-software/compare/ea7a40b61917f78493c59920a2e691d56a096691...master

Changes that are of interest to us:

1. libxml2 bumped to version with security fixes
2. postgresql bumped to minor version with security fixes
3. No need to provide md5 for ruby 2.1.6
4. Upgraded omnibus-ctl version. No big changes between versions
5. Newer version of redis, no need to pin anymore
6. Default zlib version changed, no need to pin anymore

Additionally in this MR:

1. Bumped chef version to 12.4.0.rc.2
2. Bumped nginx version to 1.7.12, contains few bugfixes
3. Updated omnibus version. Apart from new experimental features and few gem updates nothing I could find that would influence us. Diff here