PTF no more

The travis network build cache (activated via commit message, for whitelisted commiters) was added a few months back. It worked well for a while, but ultimately 
it's seen little use in practive by commiters. 

While it was gratifying to see this actually worked, all the additional
magic in ci/ doesn't pay for itself in practice, so I've decided to retire this
unless there are strong objections and get a simpler ci/ in return.

The build cache system got an upgrade as a side-effect of this 
work and `scripts/use_build_cache.py` remains a useful bit of kit.
Not entirely wasted effort after all.

xref https://github.com/pydata/pandas/pull/3383