travis: avoid using unset llvm-config

Fixes the following errors:
  usage: which [-as] program ...
  /Users/travis/.travis/job_stages: line 110: --version: command not found

... caused by the use of an undefined $LLVM_CONFIG

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
1 file changed