Fix Travis failures

Argon2 by default runs Travis CI in a containerized environment
https://docs.travis-ci.com/user/reference/overview/#Container-based

This is faster and works fine for most things. However, P-H-C#89 added ASAN
to the test cases. As ASAN requires the SYS_PTRACE capability, when
Travis/Docker permissions changed earlier this year, our tests stoped
working. See travis-ci/travis-ci#9033

The solution is just to run ASAN in a VM (sudo=true). This test takes so
long normally that the cost to startup a VM is relativly small.

.travis.yml

@@ -8,6 +8,17 @@ os:
   - linux
   - osx
 
+# Clang on Linux needs to run in a VM to use ASAN.
+# See: https://github.com/travis-ci/travis-ci/issues/9033
+matrix:
+  exclude:
+    - compiler: clang
+      os: linux
+  include:
+    - compiler: clang
+      os: linux
+      sudo: true
+
 script: make && make testci
 
 after_success: