Deb: Fix executable bit so dh-exec works (regression in fd2dc9c3)

Package contents before this fix:

libmariadb-dev-compat
drwxr-xr-x root/root ./
drwxr-xr-x root/root ./usr/
drwxr-xr-x root/root ./usr/bin/
lrwxrwxrwx root/root ./usr/bin/mysql_config
drwxr-xr-x root/root ./usr/include/
lrwxrwxrwx root/root ./usr/include/mysql
drwxr-xr-x root/root ./usr/lib/
drwxr-xr-x root/root ./usr/lib/${DEB_HOST_MULTIARCH}/
drwxr-xr-x root/root ./usr/lib/${DEB_HOST_MULTIARCH}/pkgconfig/
lrwxrwxrwx root/root ./usr/lib/${DEB_HOST_MULTIARCH}/pkgconfig/mysqlclient.pc
drwxr-xr-x root/root ./usr/lib/x86_64-linux-gnu/
lrwxrwxrwx root/root ./usr/lib/x86_64-linux-gnu/libmysqlclient.a
lrwxrwxrwx root/root ./usr/lib/x86_64-linux-gnu/libmysqlclient_r.a
lrwxrwxrwx root/root ./usr/lib/x86_64-linux-gnu/libmysqlclient_r.so
lrwxrwxrwx root/root ./usr/lib/x86_64-linux-gnu/libmysqlclient.so
drwxr-xr-x root/root ./usr/share/
drwxr-xr-x root/root ./usr/share/doc/
drwxr-xr-x root/root ./usr/share/doc/libmariadb-dev-compat/
-rw-r--r-- root/root ./usr/share/doc/libmariadb-dev-compat/changelog.gz
-rw-r--r-- root/root ./usr/share/doc/libmariadb-dev-compat/copyright

With this fix the '/usr/lib/${DEB_HOST_MULTIARCH}/' becomes correctly
populated with the actual architecture dependent path.

Also add dh-exec to Travis-CI testing so that CI passes.

As an extra small thing, clean away excess executable bit from Mroonga
plugin maintainer script that does not need it.

.travis.yml

@@ -86,7 +86,7 @@ matrix:
       addons: 
         apt:
           sources: *add-sources
-          packages: [ *common-packages, debhelper, dh-apparmor, dpatch, po-debconf, libzmq-dev, libdistro-info-perl, devscripts, fakeroot, dh-systemd, libsystemd-dev, libzstd-dev, unixodbc-dev] # make sure these match debian/control contents
+          packages: [ *common-packages, debhelper, dh-apparmor, dh-exec, dpatch, po-debconf, libzmq-dev, libdistro-info-perl, devscripts, fakeroot, dh-systemd, libsystemd-dev, libzstd-dev, unixodbc-dev] # make sure these match debian/control contents
       script:
         - ${CC} --version ; ${CXX} --version
         - env DEB_BUILD_OPTIONS="parallel=4" debian/autobake-deb.sh;
@@ -99,7 +99,7 @@ matrix:
       addons: 
         apt:
           sources: *add-sources
-          packages: [ *common-packages, debhelper, dh-apparmor, dpatch, po-debconf, libzmq3-dev, libdistro-info-perl, devscripts, fakeroot, dh-systemd, libsystemd-dev, libzstd-dev, unixodbc-dev] # make sure these match debian/control contents
+          packages: [ *common-packages, debhelper, dh-apparmor, dh-exec, dpatch, po-debconf, libzmq3-dev, libdistro-info-perl, devscripts, fakeroot, dh-systemd, libsystemd-dev, libzstd-dev, unixodbc-dev] # make sure these match debian/control contents
       script:
         - ${CC} --version ; ${CXX} --version
         - env DEB_BUILD_OPTIONS="parallel=4" debian/autobake-deb.sh;