travis: Do `make distcheck` for autotools build

.travis.yml

@@ -55,7 +55,8 @@ before_script:
   - if [ "$CI_BUILD" = "autotools" ]; then ./configure --enable-werror --with-mruby --with-neverbleed LIBSPDYLAY_CFLAGS="-I$SPDYLAY_HOME/lib/includes" LIBSPDYLAY_LIBS="-L$SPDYLAY_HOME/lib/.libs -lspdylay" CPPFLAGS=-fsanitize=address LDFLAGS=-fsanitize=address; fi
   - if [ "$CI_BUILD" = "cmake" ]; then cmake -DENABLE_WERROR=1 -DWITH_MRUBY=1 -DWITH_NEVERBLEED=1 -DSPDYLAY_INCLUDE_DIR="$SPDYLAY_HOME/lib/includes" -DSPDYLAY_LIBRARY="$SPDYLAY_HOME/lib/.libs/libspdylay.so"; fi
 script:
-  - make check
+  - if [ "$CI_BUILD" = "autotools" ]; then make distcheck; fi
+  - if [ "$CI_BUILD" = "cmake" ]; then make check; fi
   # As of April, 23, 2016, golang http2 build fails, probably because
   # the default go version is too old.
   # - cd integration-tests