Add docbook-xsl as a dependency

.travis.yml

@@ -10,7 +10,7 @@ compiler:
 before_install:
  - sudo apt-get update -qq
 install:
- - sudo apt-get install -y xsltproc zlib1g-dev libncursesw5-dev
+ - sudo apt-get install -y xsltproc docbook-xsl zlib1g-dev libncursesw5-dev
  - sudo apt-get install -y libgtk-3-dev libpango1.0-dev
 before_script:
  - mkdir build

README

@@ -18,8 +18,9 @@ this regard.
 
 Building and Running
 --------------------
-Build dependencies: CMake, pkg-config, xsltproc, ncursesw, zlib, glib-2.0,
-                    pango, termo (included), gtk+ (optional, any version)
+Build dependencies: CMake, pkg-config, xsltproc, docbook-xsl,
+                    ncursesw, zlib, termo (included),
+                    glib-2.0, pango, gtk+ (optional, any version)
 
  $ git clone https://github.com/pjanouch/sdtui.git
  $ git submodule init