Test flake: Catalog price should reject invalid values

[php-coder]

Test "Catalog price should reject invalid values" fails time to time with the following error:

Catalog price should reject invalid values :: Verify that fields w... | FAIL |
You may only interact with visible elements
Build info: version: '2.53.1', revision: 'a36b8b1cd5757287168e54b817830adce9b0158d', time: '2016-06-30 19:26:09'
System info: host: 'testing-gce-62588dc5-26bb-4572-b0e9-4b10837680df', ip: '127.0.1.1', os.name: 'Linux', os.arch: 'amd64', os.version: '3.13.0-103-generic', java.version: '1.8.0_111'
Driver info: driver.version: unknown

Log: https://travis-ci.org/php-coder/mystamps/jobs/209513814

[php-coder]

Arghhh: https://travis-ci.org/php-coder/mystamps/jobs/211069474

[php-coder]

Yet another: https://travis-ci.org/php-coder/mystamps/jobs/211948655

[php-coder]

It has started to fail again with a bit different error:

Catalog price should reject invalid values :: Verify that fields w... | FAIL |
The xpath expression '//*[@id='gibbonsPrice']' cannot be evaluated
For documentation on this error, please visit: http://seleniumhq.org/exceptions/invalid_selector_exception.html
Build info: version: '2.53.1', revision: 'a36b8b1cd5757287168e54b817830adce9b0158d', time: '2016-06-30 19:26:09'
System info: host: 'testing-gce-701205ac-7c4e-4d03-acc4-b40ef1266483', ip: '127.0.1.1', os.name: 'Linux', os.arch: 'amd64', os.version: '3.13.0-103-generic', java.version: '1.8.0_111'
Driver info: driver.version: HtmlUnitDriver

Seen here: Seen here: https://travis-ci.org/php-coder/mystamps/jobs/220069537