Disable HHVM mysqli and pgsql tests

mysqli and PostGre support isn't going to be in a good state for a
while.
diff --git a/.travis.yml b/.travis.yml
index 27fe3c6..718e6aa 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -26,6 +26,11 @@
 matrix:
   allow_failures:
     - php: hhvm
+  exclude:
+    - php: hhvm
+      env: DB=mysqli
+      env: DB=pgsql
+      env: DB=pdo/pgsql
 
 branches:
   only: