Remove capybara-webkit as dependency

[lym]

This PR removes capybara-webkit as a project dependency. This is because the gem doesn't appear to be used any where in the test code.

This commit also add imagemagick to the README because it is a project dependency.

It addresses:

• https://github.com/DefactoSoftware/Hours/issues/489
• https://github.com/DefactoSoftware/Hours/issues/488