Disable parallel build due to old sphinx.

The sphinx version on git.varnish-cache.org is v1.1.8 which
hasn't got -j.

In sphinx v1.2.2 (debian jessie) it is in place.

Guess we'll have to live with those extra 820 milliseconds
per build for now.

doc/sphinx/Makefile.am

@@ -3,7 +3,7 @@
 
 # You can set these variables from the command line.
 SPHINXOPTS    =
-SPHINXBUILD   = sphinx-build -j4 -W -N -n
+SPHINXBUILD   = sphinx-build -W -N -n
 PAPER         = a4
 BUILDDIR      = build