Add the flog package as a dependency and document it in README.

In commit 3554ff652 in the bridgedb-admin repo, we changed the shell
redirection of potential Python errors to go through the flog program,
in order to handle closing and reopening file descriptors when the
bridgedb process receives a SIGHUP.  See:

https://gitweb.torproject.org/project/bridges/bridgedb-admin.git/commit/?id=3554ff652249192fc0e6a05154e6a57d2dba8ee3

 * FIXES #19691: https://bugs.torproject.org/19691

README.rst

@@ -88,6 +88,9 @@ BridgeDB requires the following OS-level dependencies:
 -  geoip-database
 -  `python-setuptools <https://pypi.python.org/pypi/setuptools>`__
 -  libjpeg-dev
+-  `flog <https://packages.debian.org/jessie/flog>`__ (only required if bridgedb
+   is invocated with the ``run-bridgedb`` `script
+   <https://gitweb.torproject.org/project/bridges/bridgedb-admin.git/tree/bin/run-bridgedb>`__)
 
 As well as any Python dependencies in the ``requirements.txt`` file.
 

scripts/install-dependencies

@@ -18,7 +18,7 @@ if [ "$EUID" != "0" ] ; then SUDO=$(which sudo); fi
 if [ "$TRAVIS" == "true" ] ; then
     DEPENDS="${DEPENDS} realpath"
 else
-    DEPENDS="${DEPENDS} python-pip"
+    DEPENDS="${DEPENDS} python-pip flog"
 fi
 
 MISSING=""