Simplify configuration by removing the --disable-tcache option, but
replace the testing for that configuration with
--with-malloc-conf=tcache:false.
Fix the thread.arena and thread.tcache.flush mallctls to work correctly
if tcache is disabled.
This partially resolves#580.
Introduces gen_travis.py, which generates .travis.yml, and updates .travis.yml
to be the generated version.
The travis build matrix approach doesn't play well with mixing and matching
various different environment settings, so we generate every build explicitly,
rather than letting them do it for us.
To avoid abusing travis resources (and save us time waiting for CI results), we
don't test every possible combination of options; we only check up to 2 unusual
settings at a time.
