Use Xenial version of odbc-postgresql package

Travis hosts use PostgreSQL PPA which is problematic for a few reasons:

1. We don't actually test in the same environment as most SOCI users,
   who would just use the standard Ubuntu repositories and not this PPA.
2. The environment is not even stable, e.g. recently the version of
   psqlodbc has changed from 12 to 13 in Travis builds, breaking them
   due to new leak reports from ASAN.
3. We don't have debug symbols for these packages as the PPA doesn't
   provide them and the ones from Ubuntu repositories don't match. This
   prevents us from even creating precise suppressions for ASAN.

Solve all these problems at once by just sticking to the stock Xenial
version, which doesn't even trigger any leak reports, so there is
nothing to suppress any more and the changes of 457ec97e (Suppress
reports about memory leaks in PostgreSQL ODBC test, 2020-03-29) can be
reverted.

.travis.yml

@@ -3,6 +3,9 @@
 #
 # Copyright (c) 2013 Mateusz Loskot <mateusz@loskot.net>
 #
+
+# When changing this, search for "xenial" in scripts/travis and update
+# references to it there too.
 dist: xenial
 
 sudo: required

scripts/suppress_odbc.txt

@@ -1,6 +0,0 @@
-# This is a suppression file for LeakSanitizer when running ODBC tests.
-
-# Unfortunately there is a leak from psqlodbca.so for which ASAN doesn't show
-# any stack at all, so we have no choice but to suppress all leaks from this
-# library, even though it could easily hide real problems.
-leak:psqlodbca.so

scripts/travis/before_install_odbc.sh

@@ -8,4 +8,4 @@ source ${TRAVIS_BUILD_DIR}/scripts/travis/common.sh
 sudo apt-get install -qq \
     tar bzip2 \
     unixodbc-dev \
-    odbc-postgresql
+    odbc-postgresql/xenial

scripts/travis/script_odbc.sh

@@ -19,4 +19,4 @@ run_make
 
 # Exclude the tests which can't be run due to the absence of ODBC drivers (MS
 # SQL and MySQL).
-LSAN_OPTIONS=suppressions=${TRAVIS_BUILD_DIR}/scripts/suppress_odbc.txt run_test -E 'soci_odbc_test_m.sql'
+run_test -E 'soci_odbc_test_m.sql'