Add test for helptags

.travis.yml

@@ -14,7 +14,9 @@ before_install:
   - sudo apt-get install libperl-dev python-dev python3-dev ruby1.9.1-dev liblua5.1-0-dev lua5.1
   - sudo pip install cpp-coveralls --use-mirrors
 
-script: sh ./ci_run.sh
+script:
+ - sh ./ci_run.sh
+ - vim --cmd "try | helptags vim/runtime/doc/ | catch | cquit | endtry" --cmd quit
 
 after_success:
   - if [ x"$COVERAGE" == "xyes" ] ; then cd vim && coveralls -b src -x .xs -e src/xxd -e src/if_perl.c --encodings utf-8 latin-1 EUC-KR; fi