doc: update branch to master

Update the remaining markdown files to refer to the master branch.
PR-URL: #1511
Reviewed-By: Jeremiah Senkpiel <[email protected]>

COLLABORATOR_GUIDE.md

@@ -119,14 +119,14 @@ $ git rebase --abort
 Checkout proper target branch
 
 ```text
-$ git checkout v1.x
+$ git checkout master
 ```
 
 Update the tree
 
 ```text
 $ git fetch origin
-$ git merge --ff-only origin/v1.x
+$ git merge --ff-only origin/master
 ```
 
 Apply external patches
@@ -138,21 +138,21 @@ $ curl -L https://github.com/iojs/io.js/pull/xxx.patch | git am --whitespace=fix
 Check and re-review the changes
 
 ```text
-$ git diff origin/v1.x
+$ git diff origin/master
 ```
 
 Check number of commits and commit messages
 
 ```text
-$ git log origin/v1.x...v1.x
+$ git log origin/master...master
 ```
 
 If there are multiple commits that relate to the same feature or
 one with a feature and separate with a test for that feature -
 you'll need to squash them (or strictly speaking `fixup`).
 
 ```text
-$ git rebase -i origin/v1.x
+$ git rebase -i origin/master
 ```
 
 This will open a screen like this (in the default shell editor):
@@ -210,7 +210,7 @@ line.
 Time to push it:
 
 ```text
-$ git push origin v1.x
+$ git push origin master
 ```
 
 ### I just made a mistake

WORKING_GROUPS.md

@@ -1,7 +1,7 @@
 # io.js Working Groups
 
 io.js Working Groups are autonomous projects created by the
-[Technical Committee (TC)](https://github.com/iojs/io.js/blob/v1.x/GOVERNANCE.md#technical-committee).
+[Technical Committee (TC)](https://github.com/iojs/io.js/blob/master/GOVERNANCE.md#technical-committee).
 
 Working Groups can be formed at any time but must be ratified by the TC.
 Once formed the work defined in the Working Group charter is the