Fix two bugs in #decompress_file.

[kapoorlakshya]

This PR addresses two issues in System#decompress_file that Appveyor caught while running the screen-recorder specs (log):

1. Undefined variable url is referenced instead of file_name when raising an error. Seems like a copy-paste error to me :).
2. When a custom install_dir relative to the project root is used instead of the default one, target resolves to the install_dir + the driver binary. For example, ..\webdrivers_bin\chromedriver.exe. This causes a problem on Windows where the absolute path now resolves to D:\<project>\webdrivers_bin\webdrivers_bin\chromedriver.exe instead of D:\<project>\webdrivers_bin\chromedriver.exe, and File.exist? returns false for this.

> Webdrivers.install_dir
=> "webdrivers_bin"

> target
=> "webdrivers_bin/chromedriver.exe"

 File.exist? target
=> false

> File.absolute_path target
=> "D:/Projects/screen-recorder/webdrivers_bin/webdrivers_bin/chromedriver.exe"

The fix is to check for the basename (*.exe) instead of the full target path. This works because Dir.pwd is the webdrivers install dir when this check is performed in System#decomress_file.

Dir.pwd
=> "D:/Projects/screen-recorder/webdrivers_bin"

File.exist? File.basename(target)
=> true