[SQL] Using safe floating-point numbers in doctest

Test code in `sql.py` tries to compare two floating-point numbers directly, and cased [build failure(s)](https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/18365/consoleFull).

[Doctest documentation](https://docs.python.org/3/library/doctest.html#warnings) recommends using numbers in the form of `I/2**J` to avoid the precision issue.
Why 1.5 and is safer than 1.1 ? I think float + 1.0 -> 2.0 will be safer.