travis-ci: parallelize documentation build

The documentation job without parallelization takes ~10min on TravisCI.
With parallelization ("--jobs=2") it takes ~6min.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ci/test-documentation.sh

@@ -9,7 +9,7 @@ make check-builtins
 make check-docs
 
 # Build docs with AsciiDoc
-make doc
+make --jobs=2 doc
 test -s Documentation/git.html
 test -s Documentation/git.xml
 test -s Documentation/git.1
@@ -17,6 +17,6 @@ grep '<meta name="generator" content="AsciiDoc ' Documentation/git.html
 
 # Build docs with AsciiDoctor
 make clean
-make USE_ASCIIDOCTOR=1 doc
+make --jobs=2 USE_ASCIIDOCTOR=1 doc
 test -s Documentation/git.html
 grep '<meta name="generator" content="Asciidoctor ' Documentation/git.html