[spec] Fix git deprecation warnings.

spec/functional/command/push_spec.rb

@@ -54,7 +54,7 @@ module Pod
         `git remote add origin #{@upstream}`
         `git remote -v`
         `git fetch -q`
-        `git branch --set-upstream master origin/master`
+        `git branch --set-upstream-to=origin/master master`
       end
 
       # prepare the spec

spec/functional/command/repo_spec.rb

@@ -18,7 +18,7 @@ module Pod
           `git remote add origin #{upstream}`
           `git remote -v`
           `git fetch -q`
-          `git branch --set-upstream master origin/master`
+          `git branch --set-upstream-to=origin/master master`
         end
         lambda { command('repo', 'update').run }.should.not.raise
       end

spec/unit/sources_manager_spec.rb

@@ -94,7 +94,7 @@ module Pod
           `git remote add origin #{upstream}`
           `git remote -v`
           `git fetch -q`
-          `git branch --set-upstream master origin/master`
+          `git branch --set-upstream-to=origin/master master`
         end
         config.repos_dir = SpecHelper.tmp_repos_path