Now that specs can handle symlinks, put atom-build in /tmp

.gitignore

@@ -2,7 +2,6 @@
 .project
 .svn
 .nvm-version
-atom-build
 atom.xcodeproj
 build
 .xcodebuild-info

Rakefile

@@ -1,5 +1,5 @@
 ATOM_SRC_PATH = File.dirname(__FILE__)
-BUILD_DIR = 'atom-build'
+BUILD_DIR = '/tmp/atom-build'
 
 desc "Build Atom via `xcodebuild`"
 task :build => "create-xcode-project" do