fix flaky tests

Python 2.6 does not handle float error well as 2.7+

Author: Davies Liu <davies.liu@gmail.com>

Closes #1910 from davies/fix_test and squashes the following commits:

7e51200 [Davies Liu] fix flaky tests

(cherry picked from commit 882da57a1c8c075a87909d516b169b624941a6ec)
Signed-off-by: Michael Armbrust <michael@databricks.com>
SPARK-2830 [MLlib]: re-organize mllib documentation

As per discussions with Xiangrui, I've reorganized and edited the mllib documentation.

Author: Ameet Talwalkar <atalwalkar@gmail.com>

Closes #1908 from atalwalkar/master and squashes the following commits:

fe6938a [Ameet Talwalkar] made xiangruis suggested changes
840028b [Ameet Talwalkar] made xiangruis suggested changes
7ec366a [Ameet Talwalkar] reorganize and edit mllib documentation

(cherry picked from commit c235b83e2782cce0626ecc403c0a67e442be52c1)
Signed-off-by: Xiangrui Meng <meng@databricks.com>