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: install lxml without compile optimisations

Use CFLAGS="-O0" as recommended by
http://lxml.de/installation.html#installation to speedup installation
times in Travis builds.

This is applied to all installations here, should maybe be limited to
lxml.

The next step wpuld be to create precompiled wheel packages to speedup
setup.

Without this change we take on average 2,5min to setup the packages for
testing. After this we shave off 1,3min i.e. setup takes on average 57s