Fix error message about missing platformName

Author: bootstraponline

lib/appium_lib/driver.rb

@@ -257,7 +257,7 @@ def initialize opts={}
       # https://code.google.com/p/selenium/source/browse/spec-draft.md?repo=mobile
       @device = @caps[:platformName]
       @device = @device.is_a?(Symbol) ? @device : @device.downcase.strip.intern if @device
-      raise "Device must be set. Not found in options: #{opts}" unless @device
+      raise "platformName must be set. Not found in options: #{opts}" unless @device
       raise 'platformName must be Android or iOS' unless [:android, :ios].include?(@device)
 
       # load common methods
@@ -547,4 +547,4 @@ def x
 # Paging in Pry is annoying :q required to exit.
 # With pager disabled, the output is similar to IRB
 # Only set if Pry is defined.
-Pry.config.pager = false if defined?(Pry)
+Pry.config.pager = false if defined?(Pry)