travis: fix libpcre in mac builds

It looks like Travis changed their Mac image and pcre is now
installed by default. In case it gets removed again, just unlink
it before re-installing so it doesn't fail on install.

.travis.yml

@@ -126,6 +126,10 @@ before_install:
   - |
     if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
         brew update
+
+        # Unlink pcre in case its already installed.
+        brew unlink pcre || true
+
         brew install pkg-config libmagic libyaml nss nspr jansson libnet lua \
             pcre hiredis
     fi