Add Texinfo as a dependency

Required to build the software guides. However, Travis also requires
it since Autotools always builds the chip8.info file.

.travis.yml

@@ -12,7 +12,7 @@ compiler: gcc
 
 before_install:
 - sudo apt-get -qq update
-- sudo apt-get install -y libsdl2-dev check
+- sudo apt-get install -y libsdl2-dev check texinfo
 - pip install --user cpp-coveralls
 before_script: ./bootstrap
 script: "./configure --enable-gcov && make && make check"