docs: use parallel builds to speedup docs generation

Using parallel builds to increase doc regeneration as well as speedup
Travis builds.

With hot filesystem speed are as follows depending on the -j value:

Processes	Duration
1	7.75s
3	5.2s
5	4.21s
8	3.88s
10	3.96s
15	3.38s
20	3.48s

Cold (12 runs)
1	20.27 (17.82-22.71)
15	17.91 (14.41-21.42)

Not as big a saving as I'd hoped for a cold start but good for doc
development.
Travis: disable unsupported DB backends and Django versions

They're known to fail and we can reenable them when support is brought
back into core.  For now they're simply making Travis take longer to
report issues.