my/m4/my_check_docbook_ns_xslt_min.m4: new

replace the AX_CHECK_DOCBOOK_XSLT_MIN macro with a MY_CHECK_DOCBOOK_NS_XSLT_MIN
macro that checks for the *namespaced* variant of the DocBook
stylesheets.
Merge branch 'master' of ../libmy
build system: switch from AX_CHECK_DOCBOOK_XSLT_MIN to MY_CHECK_DOCBOOK_NS_XSLT_MIN

AX_CHECK_DOCBOOK_XSLT_MIN checks that the non-namespaced docbook
stylesheets are available, while the makefile was building using the
namespaced stylesheets. replace the autoconf check with one that
corresponds to the actual (namespaced) stylesheets that are used in the
build.

(fixes #1)
.travis.yml: don't install docbook-xsl

docbook-xsl is no longer necessary now that issue #1 is fixed.