Reducing the number of Travis CI Build Jobs

["James E. King, III" <jk...@apache.org>]

I'm looking at the Travis CI Build Jobs and they are taking a really long
time.  I would like to discuss paring down the build jobs in the following
ways:

1. We don't need to run cross tests on both debian and on ubuntu,
especially since we're using a dockerfile that's practically identical
between the two; one derives from the other.  I'd like to recommend
removing build jobs #2 and #4 (debian) and keeping the ubuntu ones.

2. Given we remove debian build jobs #2 and #4 above, we can keep #10 which
is a Debian build job with cmake, and eliminate #9 which is the same as #10
but on Ubuntu.  I think it is adqeuately covered in other jobs.

3. We don't need to build the matrix of (boost threads, std
threads)x(gcc,clang).  Let's get rid of #12 which is boost threads with
clang, and let's get rid of #15 which is std threads with gcc.

4. Do we need all four distribution build jobs #20 through #23?

This should reduce build time by at least 25%.

Thoughts?