Add support for WD_CHROME_PATH.

[kapoorlakshya]

• Adds support for WD_CHROME_PATH environment variable. WD_CHROME_PATH takes precedence over Selenium::WebDriver::Chrome.path. Closes Can we have a WD_CHROME_PATH ENV variable that sets chrome binary path?  #137.

Other changes:

• WD_INSTALL_DIR now has the highest precedence when determining the driver install (download) location. This is to make the behavior consistent with WD_CACHE_TIME and WD_CHROME_PATH.
• Removed ENV handing logic from the rake tasks. They now rely on the core code to handle these.

UPDATE:

• Fix bug where ENV vars took precedence over user defined configs in their project. The new order is:

1. User defined config
2. User defined ENV var value
3. Default value

• Fixes Fails to start chrome on Mac because the filename contains spaces #139 and all other file path escaping issues by switching to IO.popen, which has OS specific path escaping logic built-in. Even handles JRuby on Windows specific issue Chrome binary cannot be located on Windows (JRuby) #130.

[twalpole]

As implemented ENV['WD_CHROME_PATH'] is preferred over Selenium::WebDriver::Chrome.path (and the equivalents for other drivers)- I think that should probably be the other way around since code setting is more deliberate than environment variable.

[kapoorlakshya]

As discussed on Slack, the current behavior of ENV taking precedence over code is now categorized as a bug. I'll update the PR tonight with the new order:

1. User defined config
2. User defined ENV var
3. Default value

This way any deliberate changes in the code will take precedence over ENV values.

[kapoorlakshya]

@twalpole PR is ready for second review.

[twalpole]

Looks good to me -- other than possibly adding location method to finder classes

[twalpole]

That looks better - 👍