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.