Update RobotFramework and integration tests to use Selenium 3

[php-coder]

• update README.md: "Integration tests: Selenium2..."
• close Bump selenium.version from 2.53.1 to 3.14.0 #940 and Bump robotframework-seleniumlibrary from 2.53.1.1 to 3.12.0.1 #960
• remove selenium2 branche (I'll review it later)
• remove selenium3 branch
• update links on https://github.com/php-coder/mystamps/wiki/integration-tests
• try to reproduce "Textfield Value Should Be" fails with NPE on input type=url MarketSquare/robotframework-seleniumlibrary-java#52
• remove "unstable" tag from the tests from Add integration test for adding series to user's collection #1084/Add integration test for removing series from user's collection #45

[php-coder]

Blocked by #530 because we also have the old tests that also must be ported to the newer Selenium.

[php-coder]

htmlunit driver isn't supported by the robotframework-seleniumlibrary 3.141.59.265
See MarketSquare/robotframework-seleniumlibrary-java#79 for details.

[php-coder]

htmlunit driver isn't supported by the robotframework-seleniumlibrary

It has been fixed in 3.141.59.2653 and doesn't block us anymore.