KUDU-1822. Update docs to build the documentation in ubuntu
KUDU-1822. Update docs to build the documentation in ubuntu

Include two packages, gem and ruby-dev, in the instruction to
build documentations for Ubutnu.

Change-Id: I474a0ffd61b4c50b0c0b5885a4e615679b4ae630
Reviewed-on: http://gerrit.cloudera.org:8080/5608
Tested-by: Kudu Jenkins
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>

cloudera/kudu,cloudera/kudu,andrwng/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,cloudera/kudu,InspurUSA/kudu,cloudera/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,EvilMcJerkface/kudu,cloudera/kudu,cloudera/kudu,cloudera/kudu,helifu/kudu,helifu/kudu,helifu/kudu,andrwng/kudu,helifu/kudu,helifu/kudu,cloudera/kudu,andrwng/kudu,helifu/kudu,InspurUSA/kudu,andrwng/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,helifu/kudu,andrwng/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,andrwng/kudu,andrwng/kudu,andrwng/kudu,cloudera/kudu,EvilMcJerkface/kudu,andrwng/kudu