[travis] remove Fedora 27 and ignore static build result

Fedora 27 and LLVM 5 are EOL, which means they won't receive new
updates (not even security patches). As such, there's no reason to keep
testing those platforms/dependencies in our CI, and removing them will
reduce our CI complexity as well as unbreak our CI.

Also, static builds are broken for some time
(https://github.com/iovisor/bpftrace/issues/266), and keeping them in
our CI will only prevent collaborators from keeping the CI healthy. We
should push for Ubuntu/Debian/Fedora static builds instead of relying on
alpine for it. For now, mark alpine static build as "allow to fail" on
Travis.

.travis.yml

@@ -12,10 +12,6 @@ matrix:
       env: BASE=alpine TYPE=Debug STATIC_LINKING=ON TEST_ARGS="--gtest_filter=-codegen.string_equal_comparison:codegen.string_not_equal_comparison"
     - name: "Static LLVM 5 Release"
       env: BASE=alpine TYPE=Release STATIC_LINKING=ON
-    - name: "LLVM 5 Debug"
-      env: BASE=fedora27 TYPE=Debug
-    - name: "LLVM 5 Release"
-      env: BASE=fedora27 TYPE=Release
     - name: "LLVM 6 Debug"
       env: BASE=fedora28 TYPE=Debug
     - name: "LLVM 6 Release"
@@ -24,6 +20,11 @@ matrix:
       env: BASE=fedora29 TYPE=Debug
     - name: "LLVM 7 Release"
       env: BASE=fedora29 TYPE=Release
+  allow_failures:
+    - name: "Static LLVM 5 Debug"
+      env: BASE=alpine TYPE=Debug STATIC_LINKING=ON TEST_ARGS="--gtest_filter=-codegen.string_equal_comparison:codegen.string_not_equal_comparison"
+    - name: "Static LLVM 5 Release"
+      env: BASE=alpine TYPE=Release STATIC_LINKING=ON
 
 script:
   - docker build -t bpftrace-builder-$BASE -f docker/Dockerfile.$BASE docker/