Travis: Allow job LINUX64_MUSL USE_OPENMP=1 to fail

See: https://github.com/xianyi/OpenBLAS/pull/1255#issuecomment-318692183

.travis.yml

@@ -87,7 +87,10 @@ jobs:
         - TARGET_BOX=LINUX64_MUSL
         - BTYPE="BINARY=64"
 
-    - <<: *test-alpine
+    # XXX: This job segfaults in TESTS OF THE COMPLEX LEVEL 3 BLAS,
+    # so it's "allowed to fail" for now (see allow_failures).
+    - &test-alpine-openmp
+      <<: *test-alpine
       env:
         - TARGET_BOX=LINUX64_MUSL
         - BTYPE="BINARY=64 USE_OPENMP=1"