Install rpm-common

Fixes `error: Unable to open /usr/lib/rpm/rpmrc for reading: No such file or
directory.` on Travis CI.

Author: jan-cerny

.travis.yml

@@ -26,6 +26,7 @@ matrix:
             - swig
             - librtmp-dev
             - xsltproc
+            - rpm-common
       before_script:
         - cd build
       script:

docs/developer/developer.adoc

@@ -151,7 +151,13 @@ that you need to have these additional packages installed:
 wget lua which procps-ng initscripts chkconfig sendmail bzip2 rpm-build
 ----
 
-and it is also required to have `sendmail` service running on the system:
+On Ubuntu 18.04, also install:
+
+----
+rpm-common
+----
+
+It is also required to have `sendmail` service running on the system:
 
 ----
 $ systemctl start sendmail.service