osx CI build job fails with iostreams related test errors

This job was newly added. Looks like there are some issues to clean up. Making it an allowed failure for now.

Failing build example:

https://travis-ci.org/boostorg/iostreams/jobs/525323051

Failing test output:

```
testing.capture-output ../../bin.v2/libs/iostreams/test/stream_state_test.test/clang-darwin-9.1/release/cxxstd-03-iso/threading-multi/visibility-hidden/stream_state_test.run
====== BEGIN OUTPUT ======
Running 12 test cases...
../../libs/iostreams/test/stream_state_test.cpp:89: error: in "stream state test/wrap_nothrow <&test_seekp>__execute": unexpected exception thrown by function(stream)
../../libs/iostreams/test/stream_state_test.cpp:72: error: in "stream state test/wrap_nothrow <&test_seekp>__execute": stream still good
../../libs/iostreams/test/stream_state_test.cpp:74: error: in "stream state test/wrap_nothrow <&test_seekp>__execute": stream did not set badbit
../../libs/iostreams/test/stream_state_test.cpp:75: error: in "stream state test/wrap_nothrow <&test_seekp>__execute": stream did not fail
../../libs/iostreams/test/stream_state_test.cpp:77: error: in "stream state test/wrap_nothrow <&test_seekp>__execute": stream does not report failure by operator !
../../libs/iostreams/test/stream_state_test.cpp:79: error: in "stream state test/wrap_nothrow <&test_seekp>__execute": stream does not report failure by operator void* or bool
../../libs/iostreams/test/stream_state_test.cpp:72: error: in "stream state test/wrap_throw <&test_seekp>__execute": stream still good
../../libs/iostreams/test/stream_state_test.cpp:74: error: in "stream state test/wrap_throw <&test_seekp>__execute": stream did not set badbit
../../libs/iostreams/test/stream_state_test.cpp:75: error: in "stream state test/wrap_throw <&test_seekp>__execute": stream did not fail
../../libs/iostreams/test/stream_state_test.cpp:77: error: in "stream state test/wrap_throw <&test_seekp>__execute": stream does not report failure by operator !
../../libs/iostreams/test/stream_state_test.cpp:79: error: in "stream state test/wrap_throw <&test_seekp>__execute": stream does not report failure by operator void* or bool
unknown location:0: fatal error: in "stream state test/wrap_throw_delayed<&test_seekp>__execute": std::__1::ios_base::failure: bad seek: unspecified iostream_category error
*** 12 failures are detected in the test module "Master Test Suite"
EXIT STATUS: 201
====== END OUTPUT ======

status=0
if test $status -ne 0 ; then
echo Skipping test execution due to testing.execute=off
exit 0
fi
"../../bin.v2/libs/iostreams/test/stream_state_test.test/clang-darwin-9.1/release/cxxstd-03-iso/threading-multi/visibility-hidden/stream_state_test" > "../../bin.v2/libs/iostreams/test/stream_state_test.test/clang-darwin-9.1/release/cxxstd-03-iso/threading-multi/visibility-hidden/stream_state_test.output" 2>&1 < /dev/null
status=$?
echo >> "../../bin.v2/libs/iostreams/test/stream_state_test.test/clang-darwin-9.1/release/cxxstd-03-iso/threading-multi/visibility-hidden/stream_state_test.output"
echo EXIT STATUS: $status >> "../../bin.v2/libs/iostreams/test/stream_state_test.test/clang-darwin-9.1/release/cxxstd-03-iso/threading-multi/visibility-hidden/stream_state_test.output"
if test $status -eq 0 ; then
cp "../../bin.v2/libs/iostreams/test/stream_state_test.test/clang-darwin-9.1/release/cxxstd-03-iso/threading-multi/visibility-hidden/stream_state_test.output" "../../bin.v2/libs/iostreams/test/stream_state_test.test/clang-darwin-9.1/release/cxxstd-03-iso/threading-multi/visibility-hidden/stream_state_test.run"
fi
verbose=0
if test $status -ne 0 ; then
verbose=1
fi
if test $verbose -eq 1 ; then
echo ====== BEGIN OUTPUT ======
cat "../../bin.v2/libs/iostreams/test/stream_state_test.test/clang-darwin-9.1/release/cxxstd-03-iso/threading-multi/visibility-hidden/stream_state_test.output"
echo ====== END OUTPUT ======
fi
exit $status
...failed testing.capture-output ../../bin.v2/libs/iostreams/test/stream_state_test.test/clang-darwin-9.1/release/cxxstd-03-iso/threading-multi/visibility-hidden/stream_state_test.run...
```