build: use gettext-h

We were using the gnulib's gettext module with tricks in
bootstrap.conf to avoid useless files.  Instead, use gnulib's
gettext-h module.

* .travis.yml: Force Gettext 0.18.3 on Trusty.
* bootstrap.conf: Use gettext-h instead of gettext.
(excluded_files): Remove.
* configure.ac (AM_GNU_GETTEXT_VERSION): Bump to 0.19.

bootstrap.conf

@@ -24,7 +24,7 @@ gnulib_modules='
   dirname
   error extensions fdl fopen-safer
   getopt-gnu
-  gettext git-version-gen gitlog-to-changelog
+  gettext-h git-version-gen gitlog-to-changelog
   gpl-3.0 hash inttypes isnan javacomp-script
   javaexec-script
   ldexpl
@@ -68,23 +68,6 @@ XGETTEXT_OPTIONS_RUNTIME=$XGETTEXT_OPTIONS'\\\
  --keyword=YY_ \\\
 '
 
-# Gettext supplies these files, but we don't need them since
-# we don't have an intl subdirectory.
-excluded_files='
-    m4/glibc2.m4
-    m4/intdiv0.m4
-    m4/intl.m4
-    m4/intldir.m4
-    m4/intmax.m4
-    m4/lcmessage.m4
-    m4/lock.m4
-    m4/longdouble.m4
-    m4/signed.m4
-    m4/uintmax_t.m4
-    m4/ulonglong.m4
-    m4/visibility.m4
-'
-
 gnulib_tool_option_extras='--symlink --conditional-dependencies --makefile-name=gnulib.mk --po-base=gnulib-po --po-domain=bison'
 
 bootstrap_post_import_hook()