updated tasks/lib/distros.rb for scienfic linux

Similar to this [issue](https://github.com/iriscouch/build-couchdb/pull/31), there is a problem with `build-couchdb/tasks/lib/distros.rb` whereby it has failed to parse `/etc/redhat-release` file. This is because the script parses versions of Scientific Linux (pre 6.0), that has a pattern

    Scientific Linux SLF release (5.0) 

However since 6.0 they have adopted the following pattern:

    Scientific Linux release 6.2 (Carbon) 

See [this] (https://cfengine.com/bugtracker/bug_view_advanced_page.php?bug_id=570)