Get rid of allowed failures in Travis CI configuration

Don't allow any failures, all the tests are supposed to pass now except
for the ones using Oracle backend which is completely broken and can't
be installed any more, so disable this one completely, it has no chance
of starting to work again on its own.

.travis.yml

@@ -23,18 +23,11 @@ env:
         - SOCI_TRAVIS_BACKEND=mysql
         - SOCI_TRAVIS_BACKEND=odbc
         - SOCI_TRAVIS_BACKEND=postgresql
-        - SOCI_TRAVIS_BACKEND=oracle CFLAGS=-m32 CXXFLAGS=-m32 LDFLAGS=-m32 WITH_BOOST=OFF
         - SOCI_TRAVIS_BACKEND=sqlite3
         - SOCI_TRAVIS_BACKEND=valgrind
 
-matrix:
-    fast_finish: true
-    allow_failures:
-        - env: SOCI_TRAVIS_BACKEND=db2
-        - env: SOCI_TRAVIS_BACKEND=valgrind
-        - env: SOCI_TRAVIS_BACKEND=odbc
-        - env: SOCI_TRAVIS_BACKEND=mysql
-        - env: SOCI_TRAVIS_BACKEND=oracle CFLAGS=-m32 CXXFLAGS=-m32 LDFLAGS=-m32 WITH_BOOST=OFF
+# This build is disabled because Oracle setup is currently broken, see #554.
+# - SOCI_TRAVIS_BACKEND=oracle CFLAGS=-m32 CXXFLAGS=-m32 LDFLAGS=-m32 WITH_BOOST=OFF
 
 addons:
     apt: