fix(mdTooltip): fix regression that broke tooltips on Firefox

[Splaktar]

Remove check for pointer-events not equal to none to resolve non-visible tooltips on Firefox.
Add Chrome & Firefox as default test runners in karma task to avoid regressions like this in the future.

Closes #3047, #3250 and #3430

[Splaktar]

I would be happy to hear if this code may have existed for a reason. If so, we'll want to add a test for whatever that reason is.

We probably don't want to be missing regressions like this on releases since they would be easily caught by the automated tests had they been run for Firefox instead of just PhantomJS. It only adds 3s to the task and there is a karma-fast task for faster development runs.

Removed Chrome as a default since it fails to execute on Travis. It's possible to add Chrome when not in Travis and remove it when in Travis if that is desirable.

[Splaktar]

This attempts to improve on the proposed PR #3447