Fix typo in build description for Ubuntu

Change-Id: Ida41dae886074e8a728438da2333c29690f4cdca
Reviewed-on: http://review.couchbase.org/40288
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Add description for Debian-7

Change-Id: I79f4e3a7e3ec6f3154abc3a0c854c6943758c898
Reviewed-on: http://review.couchbase.org/40289
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Chris Hillery <ceej@couchbase.com>