Avoid call to deprecated ST_Distance_Sphere

[mkrogemann]

Switch to using ST_DistanceSphere instead. This avoids frequent warnings
in the postgresql server log.

[mkrogemann]

Just noticed that this PR already exists ;-)

[bryanjos]

@mkrogemann I can accept this PR over that one if you do the following:

• revert the change to st_distance_sphere/2
• create a new macro, st_distancesphere/2

The reason for keeping the old one is for anyone using a version of postgis that doesn't have the new function (for instance, doesn't look like travis ci's version of postgis has it). Since the other one throws a postgresql warning, I don't think we will have to make any warnings here. But maybe add a doc on st_distance_sphere/2 that it is deprecated as of postgis 2.2 I think?

[mkrogemann]

Sounds good. I will look into it

[mkrogemann]

Looks like Postgis 2.2 has been a requirement from PostgreSQL 9.5 onwards and that 2.1. is no longer supported with PostgreSQL >= 9.5.
Travis supports 9.5 and 9.6 so Postgis 2.2 should be available. Will verify.

travis-ci/travis-ci#5575

[mkrogemann]

All tests are green now that I switched to postgresql 9.6 which also required switching to the 'trusty' build environment which is (I believe) becoming the next default environment in Travis CI.
I did not add a test to cover the old macro. Would be easy to do but I felt this does not provide real benefit right now and that we should be testing the new API instead.
What is your view on this?

[bryanjos]

Looks good! Thanks!