travis: macos: unlink all deps, then relink

Kind of ugly, but first unlink all dependencies then install.
The deps that don't get an upgrade will remain unlinked, so
relink all dependencies as relinking an already linked dep
does not error out.

.travis.yml

@@ -127,12 +127,29 @@ before_install:
     if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
         brew update
 
+        # Unlink and install each dependency to avoid errors where a
+        # formula might already be installed but is not the current
+        # version.
+        packages="pkg-config libmagic libyaml nss nspr jansson libnet lua pcre hiredis"
+        for package in $packages; do
+            if brew ls $package --versions > /dev/null; then
+                brew unlink $package
+            fi
+        done
+
+        # Now install.
+        brew install $packages
+
+        # Now relink, becuase if a newer version of a package wasn't
+        # installed above, it will remain unlinked.
+        brew link $packages
+
         # Unlink pcre in case its already installed.
-        brew unlink pcre || true
+        # brew unlink pcre || true
-        brew unlink pkg-config || true
+        # brew unlink pkg-config || true
 
-        brew install pkg-config libmagic libyaml nss nspr jansson libnet lua \
+        # brew install pkg-config libmagic libyaml nss nspr jansson libnet lua \
-            pcre hiredis
+        #     pcre hiredis
     fi
   - ./qa/travis-libhtp.sh