Updated Travis configuration and categories guidelines.

Travis upgraded the build system including xctool.
http://blog.travis-ci.com/2014-11-03-xcode-61-beta/

Guidelines additions and Travis fix.

.travis.yml

@@ -1,10 +1,5 @@
-before_install: 
-- brew update
-- brew unlink xctool
-- brew install xctool
-
 language: objective-c
-xcode_sdk: iphonesimulator8.0
+xcode_sdk: iphonesimulator8.1
 install: true
 xcode_workspace: Signal.xcworkspace
 xcode_scheme: Signal

CONTRIBUTING.md

@@ -13,6 +13,8 @@ We are trying to follow the [GitHub code conventions for Objective-C](https://gi
 
 In addition to that, always add curly braces to your `if` conditionals, even if there is no `else`. Booleans should be declared according to their Objective-C definition, and hence take `YES` or `NO` as values.
 
+Any category extension on UIKit, or popular libraries, should be prefixed with `ows_` to avoid collisions.
+
 One note, for programmers joining us from Java or similar language communities, note that [exceptions are not commonly used for errors that may occur in normal use](http://stackoverflow.com/questions/324284/throwing-an-exception-in-objective-c-cocoa/324805#324805) so familiarize yourself with **NSError**.
 
 ### UI conventions