Adding FireFox to the run, but don't make the build fail.

https://travis-ci.org/mithro/web-animations-js/jobs/9140303

Hits these two issues at the moment;
```
test-bounds:Page contains no errors ... ok
test-bounds:Start bound should be inclusive at t=0s ... FAIL
test-bounds:End bound should be inclusive at t=1s ... FAIL
test-bounds:Start bound should include startDelay at t=0s ... FAIL
test-bounds:Start bound with startDelay should be inclusive at t=1s ... FAIL
test-bounds:End bound with startDelay should be inclusive at t=2s ... FAIL
```
--
```
======================================================================
FAIL: unit-test-testharness:assert_styles reference_element
----------------------------------------------------------------------
message: {{{u'computedObjectStyle is null(stack: _assert_style_element@http://localhost:56022/test/bootstrap.js:188\nassert_styles@http://localhost:56022/test/bootstrap.js:367\n@http://localhost:56022/test/testcases/unit-test-testharness.html#message:24\nTest.prototype.step@http://localhost:56022/test/testharness/testharness.js:1041\ntest@http://localhost:56022/test/testharness/testharness.js:402\n@http://localhost:56022/test/testcases/unit-test-testharness.html#message:31\n)'}}}
======================================================================
FAIL: unit-test-testharness:assert_styles should handle negative values
----------------------------------------------------------------------
message: {{{u'computedObjectStyle is null(stack: _assert_style_element@http://localhost:56022/test/bootstrap.js:188\nassert_styles@http://localhost:56022/test/bootstrap.js:367\n@http://localhost:56022/test/testcases/unit-test-testharness.html#message:71\nTest.prototype.step@http://localhost:56022/test/testharness/testharness.js:1041\ntest@http://localhost:56022/test/testharness/testharness.js:402\n@http://localhost:56022/test/testcases/unit-test-testharness.html#message:72\n)'}}}
Ran 123 tests in 52.318s
```

Then Selenium dies thanks to a Modal dialog :/