version 1.7.4 - Tenth release to CRAN

RSelenium 1.7.4

• executeScript now passes a dummy argument
• Defunct phantom() function
• Updated unit tests and test environment
• Updated vignettes and documentation

RSelenium 1.7.3

• Address issue with user/pass credentials being exposed using SauceLabs (thanks @jstockwin #131)
• Cache packages on TRAVIS to reduce runtime (thanks @jstockwin #132)

RSelenium 1.7.2

• Fixed issue where rsDriver client when failing to open didnt catch error
• Correctly pass the check argument in rsDriver to wdman (thanks @bourdieu #123)