Check for the ronn command in configure before building the man files.

.travis.yml

 before_install:
   - sudo apt-get update -qq
   - sudo apt-get install -qq libpulse-dev portaudio19-dev libsonic-dev
-  - sudo apt-get install -qq ruby
-  - sudo gem install ronn
 
 script:
   - ./autogen.sh

Makefile.am

 endif
 
 bin_PROGRAMS += src/speak-ng
+
+if HAVE_RONN
 man1_MANS += src/speak-ng.1
+endif
 
 src_speak_ng_LDADD   = src/libespeak-ng.la
 src_speak_ng_LDFLAGS = -static
 src_speak_ng_SOURCES = src/speak-ng.c
 
 bin_PROGRAMS += src/espeak-ng
+
+if HAVE_RONN
 man1_MANS += src/espeak-ng.1
+endif
 
 src_espeak_ng_LDADD   = src/libespeak-ng.la
 src_espeak_ng_CFLAGS  = -Isrc/include/espeak-ng

README.md

 To build the documentation, you need:
 
 1.  the `kramdown` markdown processor.
-2.  the `ronn` man-page markdown processor.
+2.  the `ronn` man-page markdown processor to build the man pages.
 
 ### Debian
 
 |---------------|------------------------------------------------------------------|
 | autotools     | `sudo apt-get install make autoconf automake libtool pkg-config` |
 | c11 compiler  | `sudo apt-get install gcc`                                       |
-| ronn          | `sudo apt-get install ruby-ronn`                                 |
 
 Optional dependencies:
 
 | portaudio 18 | `sudo apt-get install libportaudio-dev` |
 | portaudio 19 | `sudo apt-get install portaudio19-dev`  |
 | sonic        | `sudo apt-get install libsonic-dev`     |
+| ronn         | `sudo apt-get install ruby-ronn`        |
 
 Documentation dependencies:
 

configure.ac

 AM_CONDITIONAL(OPT_MBROLA, [test x"$have_mbrola" = xyes])
 AM_CONDITIONAL(OPT_ASYNC,  [test x"$have_async"  = xyes])
 
+dnl ================================================================
+dnl ronn checks.
+dnl ================================================================
+
+AC_CHECK_PROG(RONN, ronn, ronn, no)
+
+AM_CONDITIONAL(HAVE_RONN, [test ! x"$RONN" = xno])
+
 dnl ================================================================
 dnl Extended dictionary checks.
 dnl ================================================================